the ferrite sintered body according to the first embodiment can be produced for example as follows . first , the starting materials are weighed and mixed so as to give the predetermined ratio of composition and obtain the raw material mixture . the weighing is normally performed at an accuracy of 1 / 1000 . as the method of mixing , for example , wet mixing using a ball mill and dry mixing using a dry mixer may be mentioned . note that the average particle diameter of the starting material is preferably 0 . 1 to 3 μm . the raw material mixture in the present embodiment contains , in addition to the iron oxide or material becoming iron oxide after sintering , at least one oxide selected from magnesium oxide , nickel oxide , copper oxide , zinc oxide , manganese oxide , lithium oxide , or materials becoming these metal oxides after sintering . note that materials becoming metal oxides after sintering include metals alone , carbonates , hydroxides , halides , etc . the composition of the ferrite composition of the present embodiment is not particularly limited . various compositions may be selected in accordance with the objective , but it is preferable that the composition contain as main ingredients fe 2 o 3 and at least one oxide selected from the group of mgo , cuo , zno , mno , nio , and li 2 o . a typical example of the ferrite composition of the present embodiment is an mg — cu — zn ferrite composition . the raw material mixture in the present embodiment may also have various types of additives added to it in addition to the above main ingredients . note that the raw material mixture in the present embodiment may contain impurity elements in the material . as such elements , b , al , si , p , ca , cr , co , na , k , s , cl , etc . may be mentioned . to keep down the power loss or effect on the magnetic characteristics , it is preferable that the ratio by weight of these elements with respect to the composition as a whole be not more than 200 ppm , but p and b have a large effect on the power loss or magnetic characteristics , so the ratio by weight of p with respect to the composition as a whole is preferably 0 to 30 ppm or the ratio by weight of b with respect to the composition as a whole is preferably 0 to 50 ppm . next , the raw material mixture is prefired to obtain a prefired material . the prefiring is performed to break down the materials by heat , homogenize the components , produce ferrite , eliminate ultra - fine particles due to sintering and grow the particle diameter to a suitable size of the particles , and convert the raw material mixture to a form suited for post processing . this prefiring is preferably performed at a temperature of 700 to 1000 ° c . normally for one to three hours . the prefiring may be performed in the air or in an atmosphere having a higher oxygen partial pressure than air or a pure oxygen atmosphere . note that when including additional ingredients in the ferrite composition , the main ingredients and additional ingredients may be mixed before the prefiring or after the prefiring . next , the prefired material is pulverized to obtain a pulverized material . the pulverization is performed to break up aggregates of the prefired material to produce a powder having a suitable sinterability . when the prefired material forms large masses , the material is roughly pulverized then wet pulverized using a ball mill or attritor mill etc . the wet pulverization is performed until the average particle diameter of the prefired material becomes preferably 1 to 2 μm . next , the pulverized material is granulated to obtain a granulated material . the granulation is performed to make the pulverized material into aggregated particles of a suitable size to convert it to a form suitable for shaping . as this granulation method , for example , the press granulation method or spray dry method etc . may be mentioned . in the present embodiment , the spray dry method is used of adding polyvinyl alcohol or another ordinarily used binder to the pulverized material , then atomizing it in a spray dryer and drying it at a low temperature . next , the granulated material is shaped to a predetermined shape to obtain a shaped article . as the shaping of the granulated material , for example , dry shaping , wet shaping , extrusion , etc . may be mentioned , but in the present embodiment , the dry shaping method of filling the granulated material into a mold and then pressing it is used . the shape of the shaped article is not particularly limited , but for example a toroidal shape etc . may be mentioned . next , the shaped article is sintered to obtain a ferrite sintered body of the composition according to the present embodiment . the sintering is performed to cause the powder particles of the shaped article including the large number of voids to become cohesive at a temperature under the melting point to obtain a dense sintered body . as the furnace used for the sintering , one of the batch type , pusher type , cart type , etc . may be mentioned . in the present embodiment , for example air or another gas is blown into the furnace during sintering by a flow rate of preferably more than 25 ml / min to less than 5000 ml / min , more preferably 200 to 4000 ml / min . if the flow rate of gas blown into the furnace is too small , the amount of carbon contained after sintering tends to become large and the mechanical strength of the ferrite sintered body obtained to become low . further , if the flow rate of the gas blown into the furnace is too large , the amount of carbon contained after sintering tends to become too small and the magnetic permeability of the obtained ferrite sintered body to become low . that is , by sintering while blowing gas into the furnace by a predetermined flow rate , it is possible to control the amount of carbon contained after sintering and balance the mechanical strength ( bending strength ) and magnetic characteristics ( magnetic permeability ) of the obtained ferrite sintered body . the sintering temperature is preferably 900 to 1300 ° c . if the sintering temperature is too low , not only does the sintering tend to become insufficient , but also the amount of carbon contained after sintering tends to become larger . the sintering time is normally about 1 to 3 hours . the sintering may be performed in the air or in an atmosphere with an oxygen partial pressure higher than air . by going through the above process , a ferrite sintered body controlled in amount of carbon to a suitable value is obtained . the average crystal particle diameter of the ferrite sintered body is preferably 1 to 30 μm . if the average crystal particle diameter is too small , the hysteresis loss tends to become large , while if the average crystal particle diameter is too large , the eddy current loss tends to become large . the ferrite sintered body of the composition according to the present embodiment can be improved in mechanical strength while maintaining a high magnetic permeability by controlling the content of carbon to within a predetermined range ( over 9 . 7 weight ppm to less than 96 weight ppm ). further , according to the process of the present embodiment , the content of the carbon is controlled by sintering while blowing gas by a predetermined flow rate into the furnace , so it is possible to produce a ferrite sintered body controlled in amount of carbon after sintering to a suitable amount simply and inexpensively . accordingly , by using this ferrite sintered body to construct an inductor magnetic core , it is possible to obtain a product superior in magnetic permeability , prevented from cracking or chipping even if thin , and superior in reliability . further , this contributes to the reduction of size and reduction of cost of the product . the ferrite sintered body according to the second embodiment may be produced for example in the following way . in the same way as the first embodiment , first , the starting materials are weighed and mixed to a predetermined ratio of composition to obtain a raw material mixture . note that the average particle diameter of the starting materials is preferably 0 . 1 to 3 μm . the composition of the ferrite composition is not particularly limited . various compositions may be selected in accordance with the objective . preferably , the ferrite composition contains , in addition to fe 2 o 3 , mno and zno . a typical example of the ferrite composition of the present embodiment is an mn — zn ferrite composition . in this case , the raw material mixture contains as main ingredients iron oxide , manganese oxide , and zinc oxide or materials becoming these metal oxides after sintering . note that materials becoming metal oxides after sintering include metals alone , carbonates , hydroxides , halides , etc . the raw material mixture in the present embodiment preferably has the following additional ingredients added to it in addition to the above main ingredients . the additional ingredients include at least one oxide selected from silicon oxide , calcium oxide , tin oxide , titanium oxide , niobium oxide , zirconium oxide , vanadium oxide , molybdenum oxide , bismuth oxide , and tantalum oxide . preferably , the additional ingredients include at least one oxide selected from silicon oxide in an amount of addition converted to sio 2 of 50 to 2000 ppm , calcium oxide in an amount of addition converted to cao of 100 to 3100 ppm , tin oxide in an amount of addition converted to sno 2 of not more than 8500 ppm ( not including 0 ), titanium oxide in an amount of addition converted to tio 2 of not more than 12000 ppm ( not including 0 ), niobium oxide in an amount of addition converted to nb 2 o 5 of 50 to 300 ppm , zirconium oxide in an amount of addition converted to zro 2 of 200 to 1200 ppm , vanadium oxide in an amount of addition converted to v 2 o 5 of 100 to 1100 ppm , molybdenum oxide in an amount of addition converted to moo 3 of 50 to 310 ppm , bismuth oxide in an amount of addition converted to bi 2 o 3 of 350 to 800 ppm , and tantalum oxide in an amount of addition converted to ta 2 o 5 of 400 to 1400 ppm . by adding these additional ingredients in this range , the br ( residual magnetic flux density ) is reduced , the δb (= bm - br ) increases , the power loss decreases , and the magnetic characteristics are improved . note that the raw material mixture in the present embodiment may include impurity elements in the materials . as such elements , b , al , p , cr , co , na , k , s , cl , etc . may be mentioned . to keep down the power loss and the effect on the magnetic characteristics , the ratio by weight of each element with respect to the composition as a whole is preferably not more than 200 ppm , but p and b have a large effect on the power loss or magnetic characteristics , so the ratio by weight of p with respect to the composition as a whole is preferably 0 to 30 ppm or the ratio by weight of b with respect to the composition as a whole is preferably 0 to 50 ppm . next , the raw material mixture is prefired , pulverized , granulated , and shaped , then sintered to obtain a ferrite shaped article in the same way as in the first embodiment . further , in the present embodiment , an atmospheric gas preferably substantially the same as the sintering atmosphere is blown into the furnace during sintering at a flow rate of preferably more than 100 ml / min and less than 5000 ml / min , more preferably 300 to 4000 ml / min . if the flow rate of the atmospheric gas blown into the furnace is too small , the amount of carbon contained after sintering tends to become large , the mechanical strength of the ferrite sintered body obtained to become low , and further the core loss to increase . further , if the flow rate of the atmospheric gas blown into the furnace is too large , the amount of carbon contained after sintering tends to become too small and the core loss of the obtained ferrite sintered body to become high . that is , by sintering while blowing atmospheric gas into the furnace by a predetermined flow rate , it is possible to control the content of carbon after sintering and possible to balance the mechanical strength ( bending strength ) and magnetic characteristics ( core loss ) of the ferrite sintered body obtained . the sintering temperature is preferably 1200 to 1400 ° c . if the sintering temperature is too low , not only does the sintering tend to become insufficient , but also the amount of carbon contained after sintering tends to become larger . the sintering time is normally about 3 to 7 hours . by going through the above process , a ferrite sintered body controlled in amount of carbon to a suitable value is obtained . the average crystal particle diameter of the ferrite sintered body is preferably 1 to 30 μm . if the average crystal particle diameter is too small , the hysteresis loss tends to become large , while if the average crystal particle diameter is too large , the eddy current loss tends to become large . the ferrite sintered body of the composition according to the present embodiment can be improved in mechanical strength while reducing the core loss by controlling the content of carbon to within a predetermined range ( over 9 . 8 weight ppm to less than 52 weight ppm ). further , according to the process of the present embodiment , the content of the carbon is controlled by sintering while blowing gas by a predetermined flow rate into the furnace , so it is possible to produce a ferrite sintered body controlled in amount of carbon after sintering to a suitable amount simply and inexpensively . accordingly , by using this ferrite sintered body to construct a transformer magnetic core , it is possible to obtain a product with a small core loss , prevented from cracking or chipping even if thin , and superior in reliability . further , this contributes to the reduction of size and reduction of cost of the product . the ferrite sintered body according to the third embodiment may be produced for example in the following way . in the same way as the first and second embodiments , first , the starting materials are weighed and mixed to a predetermined ratio of composition to obtain a raw material mixture . note that the average particle diameter of the starting materials is preferably 0 . 1 to 3 μm . the raw material mixture in the present embodiment contains , in addition to the iron oxide and nickel oxide or material becoming these oxides after sintering , at least one oxide selected from copper oxide , zinc oxide , and manganese oxide or materials becoming these metal oxides after sintering . note that materials becoming metal oxides after sintering include metals alone , carbonates , hydroxides , halides , etc . the composition of the ferrite composition is not particularly limited . various compositions may be selected in accordance with the objective . preferably , the composition contains as main ingredients fe 2 o 3 , nio , and at least one oxide selected from cuo , zno , and mno . a typical example of the ferrite composition of the present embodiment is an ni — cu — zn ferrite composition . the raw material mixture in the present embodiment may have added to it various types of additives in addition to the above main ingredients . note that the raw material mixture in the present embodiment may include impurity elements in the materials . as such elements , b , al , si , p , ca , cr , co , na , k , s , cl , etc . may be mentioned . to keep down the power loss and the effect on the magnetic characteristics , the ratio by weight of each element with respect to the composition as a whole is preferably not more than 200 ppm , but p and b have a large effect on the power loss or magnetic characteristics , so the ratio by weight of p with respect to the composition as a whole is preferably 0 to 30 ppm or the ratio by weight of b with respect to the composition as a whole is preferably 0 to 50 ppm . next , the raw material mixture is prefired , pulverized , granulated , and shaped , then sintered to obtain a ferrite shaped article in the same way as in the first embodiment . in the present embodiment , for example air or another gas is blown into the furnace during sintering by a flow rate of preferably more than 100 ml / min and less than 5000 ml / min , more preferably 200 to 4000 ml / min . if the flow rate of the gas blown into the furnace is too small , the content of the carbon after sintering becomes larger and the mechanical strength of the ferrite sintered body obtained ends to become low . further , if the flow rate of gas blown into the furnace becomes too large , the content of carbon after sintering tends to become too small and the magnetic permeability of the ferrite sintered body obtained to become low . that is , by sintering the material while blowing gas into the furnace by a predetermined flow rate , it is possible to control the amount of carbon after sintering and possible to balance the mechanical strength ( bending strength ) and magnetic characteristics ( magnetic permeability ) of the ferrite sintered body obtained . note that the sintering temperature , sintering time , and sintering atmosphere may be made the same as in the first embodiment . by going through the above process , a ferrite sintered body controlled in amount of carbon to a suitable value is obtained . the average crystal particle diameter of the ferrite sintered body is preferably 1 to 30 μm . if the average crystal particle diameter is too small , the hysteresis loss tends to become large , while if the average crystal particle diameter is too large , the eddy current loss tends to become large . the ferrite sintered body of the composition according to the present embodiment can be improved in mechanical strength while maintaining a high magnetic permeability by controlling the amount of carbon to within a predetermined range ( over 9 . 7 weight ppm to less than 60 weight ppm ). further , according to the process of the present embodiment , the content of the carbon is controlled by sintering while blowing gas by a predetermined flow rate into the furnace , so it is possible to produce a ferrite sintered body controlled in amount of carbon after sintering to a suitable amount simply and inexpensively . accordingly , by using this ferrite sintered body to construct an inductor magnetic core , it is possible to obtain a product superior in magnetic permeability , prevented from cracking or chipping even if thin , and superior in reliability . further , this contributes to the reduction of size and reduction of cost of the product . next , the present invention will be explained in further detail by more specific examples , but the present invention is not limited to these examples . as the materials , 48 . 0 mol % of fe 2 o 3 , 19 . 3 mol % of mgo , 7 . 1 mol % of cuo , and 25 . 6 mol % of zno were weighed , then wet mixed by a ball mill for 16 hours to obtain a raw material mixture . next , the raw material mixture was prefired at 900 ° c . for 2 hours to obtain a prefired material , then was wet pulverized by a ball mill for 16 hours to obtain a pulverized material . next , 10 wt % of an 6 % aqueous solution of polyvinyl alcohol was added as a binder to 100 wt % of the pulverized material and granulated to obtain a granulated material . this was pressed at a pressure of 1 ton / cm 2 to a toroidal shape to obtain a shaped article . further , for the test of the bending strength , the material was pressed at a pressure of 1 ton / cm 2 to a rod shape of 5 × 5 × 500 mm to obtain a shaped article . next , these shaped articles were placed in a sintering furnace for sintering . this sintering was performed by blowing air inside the furnace while changing the flow rate of blowing as shown in table 1 ( unit : ml / min ) in an air atmosphere at a sintering temperature of 1020 ° c . for a sintering time of 2 hours to obtain ferrite sintered bodies . the amount of carbon of each ferrite sintered body obtained in this way ( unit : ppm ) was measured . the amount of carbon was measured using a carbon - sulfur analyzer ( emia520 ) made by horiba seisakusho , sintering the sample by high frequency heating in a flow of oxygen , and measuring by the infrared absorption method . the results are shown in table 1 . further , the magnetic permeability μ of each obtained ferrite sintered body was measured . the magnetic permeability μ was found by winding a copper wire ( wire diameter : 0 . 35 mm ) around a sample 20 turns , measuring the inductance at a measurement frequency of 100 khz and measurement current of 0 . 25 ma by an lcr meter ( made by hewlett - packard ), and calculating using the following equation ( 1 ). the results are shown in table 1 . the relationship with the amount of carbon is shown in fig1 . magnetic permeability μ =( le × l )( μ 0 × ae × n 2 ) ( 1 ) here , le is the length of the magnetic path , l is the inductance of the sample , μ 0 is the magnetic permeability in vacuum = 4n × 10 − 7 ( h / m ), ae is the sectional area of the sample , and n is the number of turns of the coil . further , the bending strength was tested in accordance with jis - r1601 using a rod - shaped ferrite sintered body . the results are shown in table 1 . the relationship with the amount of carbon is shown in fig2 . from the above results , it was confirmed that if the amount of the carbon after sintering is 96 ppm ( comparative example 1 ), the bending strength is a low 7 . 8 kgf / mm 2 and the ferrite shaped article lacks reliability . as opposed to this , when the amount of carbon after sintering is not more than 91 ppm ( examples 1 to 7 and reference example 1 ), the bending strength is a sufficiently large 8 kgf / mm 2 to 16 . 5 kgf / mm 2 and the magnetic permeability μ is a sufficiently large 734 to 845 . among these , examples 2 to 7 were well balanced in bending strength and magnetic permeability . note that it was confirmed that when the amount of carbon after sintering is 9 . 7 ppm ( reference example 1 ), while the magnetic permeability tends to fall somewhat , there is not that much of a problem in practice . [ 0092 ] 53 . 6 mol % of fe 2 o 3 , 36 . 2 mol % of mno , and 10 . 2 mol % of zno were weighed , then wet mixed by a ball mill for 16 hours to obtain a raw material mixture . next , the raw material mixture was prefired at 850 ° c . to obtain a prefired material , then 110 weight ppm of sio 2 powder , 500 weight ppm of cao powder , 3300 weight ppm of sno 2 powder , 320 weight ppm of nb 2 o 5 powder , and 100 weight ppm of moo 3 powder were added to 100 wt % of such a prefired material , then the results were wet pulverized by a ball mill for three hours to obtain a pulverized material . next , 10 wt % of an 8 % aqueous solution of polyvinyl alcohol was added as a binder to 100 wt % of the pulverized material and the result granulated to obtain a granulated material . this was pressed in the same way as in examples 1 to 7 to obtain a toroidal shaped article and rod - shaped article . next , these shaped articles were placed in a sintering furnace for sintering . this sintering was performed by blowing a sintering atmosphere gas controlled in oxygen partial pressure inside the furnace while changing the flow rate of blowing as shown in table 2 ( unit : ml / min ) in an atmosphere controlled in oxygen partial pressure at a sintering temperature of 1300 ° c . for a sintering time of 5 hours to obtain ferrite sintered bodies . the amounts of carbon of the obtained ferrite sintered bodies were measured in the same way as in examples 1 to 7 . the results are shown in table 2 . further , a sine wave alternating current magnetic field of 100 khz and 200 mt was applied to the obtained ferrite sintered body based on jis - c2561 - 1992 and the core loss at 100 ° c . ( unit : kw / m 3 ) was measured . the results are shown in table 2 . the relationship with the amount of carbon is shown in fig3 . further , the same procedure was followed as in examples 1 to 7 to test the bending strength using the rod shaped ferrite sintered body . the results are shown in table 2 . the relationship with the amount of carbon is shown in fig4 . from the above results , it was confirmed that when the amount of carbon after sintering is 52 ppm ( reference example 2 ), the bending strength is a relatively low 5 . 7 kgf / mm 2 . as opposed to this , when the amount of carbon after sintering is not more than 42 ppm ( examples 8 to 11 and reference example 3 ), the bending strength is a sufficiently large 9 . 7 kgf / mm 2 to 20 . 2 kgf / mm 2 and the core loss is also a sufficiently low 368 kw / m 3 to 433 kw / m 3 . among these , examples 8 to 11 were well balanced in bending strength and core loss . note that it was confirmed that when the amount of carbon after sintering is 9 . 8 ppm ( reference example 3 ), the core loss tends to become somewhat greater , but this is not that much of a problem in practice . the same procedure was followed as in examples 1 to 7 to obtain toroidal shaped articles and rod shaped articles except for using 49 . 5 mol % of fe 2 o 3 , 9 mol % of nio , 10 mol % of cuo , and 31 . 5 mol % of zno as materials . next , the shaped articles were placed in the sintering furnace for sintering . the sintering was performed by blowing air into the furnace while changing the flow rate of blowing as shown in table 3 ( unit : ml / min ) in an air atmosphere at a sintering temperature of 1100 ° c . and a sintering time of 2 hours to obtain ferrite sintered bodies . the amount of carbon of each ferrite sintered body found in this way was measured in the same way as in examples 1 to 7 . the results are shown in table 3 . further , the magnetic permeability μ of each obtained ferrite sintered body was measured in the same way as in examples 1 to 7 . the results are shown in table 3 . the relationship with the amount of carbon is shown in fig6 . further , the bending strength was tested in the same way as in examples 1 to 7 using the rod - shaped ferrite sintered bodies . the results are shown in table 3 . the relationship with the amount of carbon is shown in fig6 . from the above results , it was confirmed that if the amount of carbon after sintering is 67 ppm ( reference example 4 ), the bending strength is a relatively low 7 . 1 kgf / mm 2 . as opposed to this , when the amount of carbon after sintering is not more than 58 ppm ( examples 12 to 16 and reference example 5 ), the bending strength is a sufficiently large 8 . 3 kgf / mm 2 to 13 . 4 kgf / mm 2 and the magnetic permeability μ is a sufficiently large 755 to 864 . among these , examples 14 to 16 were well balanced in the bending strength and magnetic permeability . note that when the amount of carbon after sintering is 9 . 7 ppm ( reference example 5 ), it was confirmed that while the magnetic permeability tends to fall somewhat , there is not that much of a problem in practice . note that above the explanation was made of embodiments and examples of the present invention , but the invention is not limited to these embodiments or examples in any way . it is of course possible to work the invention in various ways within the range of the gist of the invention .