hereinbelow , the embodiments of the present invention will be described in detail with reference to the drawings . in the following description , however , a dimension , a material and a shape of respective constituent members , their relative arrangement , etc ., may be appropriately modified and should be understood that the scope of the present invention is not restricted thereto unless otherwise specified . [ 0047 ] fig7 shows a general structure of an image forming apparatus according to this embodiment . referring to fig7 this image forming apparatus 51 ( a laser beam printer in this embodiment ) is an apparatus in which a latent image is formed on an electrophotographic photosensitive member 1 in the form of a drum ( hereinafter referred to as a “ photosensitive drum ”) by irradiating the photosensitive drum 1 with a laser beam based on image data by an optical system 52 , and the latent image is developed with a toner ( not shown ) to form a toner image on the photosensitive drum 1 . meanwhile , a sheet material s is placed in a sheet feeding cassette 55 and is conveyed to a main assembly of the apparatus 51 by picking up the sheet material s sheet by sheet by means of a sheet feeding mechanism including a pickup roller 56 , a feed roller 57 and a retard roller 58 . the sheet material s fed by the feeding mechanism is conveyed from a pair of conveyer rollers 59 to a pair of resister rollers 60 to align the sheet material s in order , and is conveyed to a transfer portion . at the transfer portion , the toner image formed on the above - mentioned photosensitive drum 1 is transferred onto the sheet material s by a transfer roller 61 as a transfer means . the sheet material s is then conveyed to a fixing means by guiding it on a guiding plate 62 . the fixing means includes a pressure roller 63 and a fixing roller 64 containing a heater , and applies heat and pressure to the sheet material s which passes through the fixing means , so that the toner image having been transferred onto the sheet material s is fixed to the sheet material s . the sheet material s is conveyed farther by a pair of discharging rollers 65 to be discharged into a discharge portion 66 through a reversing path . incidentally , the image forming apparatus 51 is capable of manually feeding the sheet material by manual feed tray and rollers ( not shown ). next , a process cartridge 67 detachably mountable to the image forming apparatus 51 described above will be explained with reference to fig6 which illustrates a general structure of the process cartridge 67 used in this embodiment . the process cartridge 67 includes a photosensitive drum 1 and at least one process means . examples of the process means may include a charging means 2 for charging the surface of the photosensitive drum 1 , a developing means ( developing apparatus ) 4 for forming a toner image on the photosensitive drum 1 , and cleaning means 11 for removing a toner remaining on the photosensitive drum 1 . as shown in fig6 the process cartridge according to this embodiment includes , around the photosensitive drum 1 , the charging member 2 , the developer device frame 12 , the developing means including the developing sleeve 5 as an image bearing member and a developing blade 9 d , and the cleaning means 11 . these process means is covered with a housing which is formed by a frame to be integrally supported to provide a cartridge , which is detachably mounted to a main assembly of the image forming apparatus ( not shown ). hereinbelow , the developing means 4 according to this embodiment will be described . the developing sleeve 5 include a magnetic roller 6 therein is rotatably mounted to the developing device frame 12 through a sleeve bearing ( not shown ). a toner supplied from the developing device frame 12 is attached to the surface of the developing sleeve 5 by a magnetic force of the magnetic roller 6 and is regulated to have a uniform thickness by the developing blade 9 d which comprises a rubber blade and a d blade plate as a supporting plate for supporting the rubber blade . thereafter , the toner is conveyed to a position opposite from a latent image on the photosensitive drum 1 by rotation of the developing sleeve 5 and is attached to the latent image to effect development . further , the developing means 4 is provided with a magnetic seal member 71 at both end portions of the developing sleeve 5 in a longitudinal direction of the developing sleeve 5 as shown in ( d ) of fig5 . the magnetic seal member 71 is disposed with a gap g with respect to the peripheral surface of the developing sleeve 5 and is mounted to the developing device frame 12 . the magnetic seal member 71 is prepared by bonding a magnetic plate ( magnetic member ) 74 to a magnet 73 at an outer side surface thereof in a width direction of the magnet 73 . next , the magnetic seal member 71 used in this embodiment will be described in detail . the magnetic seal member 71 has a shape as shown in fig1 . referring to fig1 the magnetic seal member 71 includes a magnet ( arcuate portion ) 73 which is an injection - molded product having a width of 3 . 4 mm and comprising a nylon binder which contains a magnetic powder of nd — fe — b , and a 0 . 6 mm - thick magnetic plate 74 formed of an sus ( stainless steel ) material . as described hereinafter , the magnetic seal member 71 includes the magnetic plate disposed at the side surface of the magnet 73 and a magnetic plate which is extended linearly from the upper ( top ) end portion and is integral with the magnetic plate on the side of the magnet 73 . these magnetic plates are formed by bending a single magnetic plate . the thus bending - treated magnetic plate and the magnet are integrally molded in one piece to constitute the magnetic seal member . the gap g ′ between a developing sleeve 5 and the magnetic seal member 71 is 0 . 1 - 0 . 7 mm ( as shown in ( d ) of fig5 ). at that time , a magnetic flux density at the surface of the developing sleeve 5 given by magnetic force of the magnetic seal member 71 is about 1000 - 2000 gs . further , with respect to a positional relationship between the magnet 73 and the magnetic plate 74 disposed at its side surface , the magnet 73 is disposed on a side closer to an opening 26 of a developing device frame 12 , and the magnetic plate 74 is disposed on a side opposite from the side closer to the opening 26 ( i . e ., an outer side surface of the magnet in a longitudinal direction of the developing sleeve ) at a portion extending from a arcuate portion 71 a of the magnetic seal member 71 to a linear portion 71 b above the arcuate portion 71 a . further , at a portion 74 c ( extending away from the peripheral surface of the developing sleeve ) located above and other than the portions to be opposite to the developing sleeve 5 , there is no magnet 73 , and as shown in fig1 the magnetic seal member 71 is formed only of the magnetic plate 74 at the portion 74 c . the portion 74 c ( other than the portions to be opposite to the developing sleeve 5 ) has a function of positioning the above - mentioned magnetic seal member 71 relative to the above - mentioned developing device frame 12 . the portion 74 c includes a bent engage portion 74 c 1 for performing the positioning of the magnetic seal member 71 in a substantially vertical direction in fig5 relative to the frame 12 ( at its upper and lower end surfaces ), and an abutting surface 74 c 2 for performing the positioning in a substantially horizontal direction in fig5 by abutment with the d blade plate 9 d 1 ( for supporting the developing blade ). the bent engage portion 74 c 1 is located in a position where it does not abut to the supporting plate 9 d 1 of the developing plate 9 d . more specifically , the bent engage portion 74 c 1 is located in a depressed position relative to the abutting surface 74 c 1 . by doing so , a positional relationship of the abutting surface 74 c 2 with other portions becomes clear , so that a surface accuracy of the abutting surface is readily ensured . in this regard , if the bent engage portion 74 c 1 and the abutting surface 74 c 2 are at the same level in the horizontal direction , it is necessary to design an angle between a flat surface 12 : of the frame 12 for positioning the supporting plate 9 d 1 of the developing blade 9 d and an upper end surface 72 g ( which determines a position of the magnetic seal member in a substantially vertical direction by abutment thereof with the upper end surface of the bent engage portion 74 c 1 ) of an engage ( mounting ) groove 72 ( of the frame 12 ) for engaging the magnetic seal member therein , to be a right angle . as a result , the magnetic seal member 71 is not readily engaged into the frame 12 . however , as in this embodiment , the bent engage portion 74 c 1 is designed to be in a depressed position relative to the abutting surface 74 c 2 , whereby it is possible to locate the upper end surface 72 g of the groove 72 away from the mounting flat surface 12 i of the developing blade 9 d . as a result , it is possible to form an oblique surface at a corner as shown in ( a ) of fig5 thus facilitating engagement of the magnetic seal member 71 with the frame 12 . further , the portions of the magnetic seal member 71 to be opposite to the developing sleeve 5 are disposed so that the magnet is on the ( inner ) side closer to the opening 26 of the developing device frame 12 and the magnetic plate 74 is on the ( outer ) side opposite from the side closer to the opening 26 . accordingly , magnetic lines 24 of force are formed between the magnet 73 and the magnetic plate 74 , as shown in fig3 and enter the magnetic plate 74 which has a high ( magnetic ) permeability . as a result , it is possible to prevent generation of magnetic lines of force extending out from a width of the magnetic seal member 71 . further , the generated magnetic lines of force are concentrated at the magnetic plate 74 , so that a resultant magnetic flux density at the surface of the magnet 73 is increased , thus creating a large magnetic force . as a result , a sealing performance of the magnetic sealing member 71 is improved . next , a method of mounting and positioning the magnetic sealing member 71 will be described . as shown in fig2 on an outer peripheral surface side ( back side ) of the magnet 73 and the magnetic plate 74 , an elastic lining 77 of an elastic material is disposed . the elastic lining 77 has a width , in a longitudinal direction of the developing sleeve 5 , which is substantially equal to a total of widths of the magnet 73 and the magnetic plate 74 . further , as shown in fig1 the elastic lining 77 is disposed so that its lower ( bottom ) end surface on an outer side in the longitudinal direction of developing sleeve 5 ( right - hand side on the figure ) covers a lower end surface 71 f of the magnetic seal member and a lower end surface 77 f on an inner side ( left - hand side on the figure ) and the lower end surface 71 f of the magnetic seal member are substantially in one plane . on the other hand , referring again to fig2 an upper ( top ) end surface 77 g of the elastic lining 77 is located at a position somewhat below an upper end surface 73 g of the magnetic plate 74 . the elastic lining 77 is applied to the backside of the magnet 73 and the magnet plate 74 with a double - faced ( adhesive ) tape . further , the developing device frame 12 is provided with a mounting groove 72 for mounting the magnetic seal member 71 , which groove extending from a flat surface 12 i to an arcuate surface 12 j as shown in fig4 . the mounting groove 72 has a shape shown in fig5 and the shape corresponds to the cross section of the magnetic seal member 71 , so that the elastic lining 77 exhibits the same compression amount over its entire area ( with respect to the arcuate portion 71 a and the linear portion 71 b of the magnetic seal member 71 and the positioning portion 74 c of the magnetic plate 74 . further , as shown in fig4 at an entrance of the mounting groove 72 ( positioning groove 72 d ) as an entering portion for the positioning portion 74 c , a relatively large c surface ( oblique surface ) is provided so as to permit easy entrance of the upper end surface ( a first positioning portion for positioning the magnetic seal member relative to the frame in a substantially vertical direction ) of the positioning portion 74 c of the magnetic seal member 71 into the upper end surface 72 g of the groove 72 . as shown in ( a ) of fig5 the magnetic seal member 71 is engaged into the mounting groove 72 ( for mounting the magnetic seal member 71 ) of the developing device frame 12 as indicated by an arrow . then , an arcuate portion 71 a of the magnetic seal member 71 is fitted into an arcuate groove 72 a , and a linear surface portion 71 b of the magnetic seal member 71 is fitted into a linear groove 72 b , as shown in ( b ) of fig5 . at this time , a boundary portion 71 n ( fig1 ) between the linear portion 71 b and the positioning portion 74 c of the magnetic plate 74 is an oblique surface , thus resulting in smooth mounting in the mounting groove 72 at the time of assembly . when the magnetic seal member 71 is lightly pressed in a direction of an arrow a , a lower portion 77 a ( fig2 ) of the elastic lining 77 is compressed . at the same time , the lower end surface 71 f of the magnetic seal member 71 presses the lower end abutting portion 72 f , and the bent engage portion 74 c 1 of the positioning portion 74 c of the magnetic seal member 71 is flush with the groove 72 d . accordingly , when the abutting surface 74 c 2 ( a second positioning portion for positioning the magnetic seal member relative to the frame in the insertion direction ) of the magnetic seal member 71 is pushed toward the rear side in the direction ( right - hand direction , i . e ., insertion direction ) crossing the arrow a , the magnetic seal member 71 is engaged with the groove 72 as shown in ( c ) of fig5 . at the time of the pressing , a heavier load than that at the time of actual operation is applied to the magnetic seal member 71 . in order to pass the bent engage portion 74 c 1 of the positioning portion 74 c of the magnetic seal member 71 to the entrance to the groove 72 d , the lower portion 77 a of the elastic lining 77 has to be compressed when compared with the time of actual operation . particularly , in the case of the conventional magnetic seal member 71 ′, the magnetic seal member 71 ′ does not enter the groove 72 d if it is further pressed downwardly from the state of ( b ) of fig1 . however , in the case of the conventional magnetic seal member , a load is liable to be applied to the magnet portion of the magnetic seal member to damage the magnetic seal member . on the other hand , in this embodiment , the bent engage portion 74 c 1 is thin and is liable to enter the groove 72 d due to the c surface provided to the entrance of the groove 72 d . in addition , the positioning portion 74 c of the magnetic seal member 71 is a thin plate of sus ( stainless steel ), thus being capable of being somewhat elastically deformed to be expected to improve an assembling performance . further , the load applied to the magnetic portion 73 of the magnetic seal member 71 is reduced when compared with the conventional magnetic seal member 71 ′. incidentally , the degree of the elastic deformation of the positioning portion 74 c of the magnetic seal member 71 is minute and the load applied to the magnetic seal member 71 becomes smaller after the mounting , so that an amount of the deformation of the positioning portion 74 c after the mounting is negligible . then , a developing blade 9 d is mounted to the developing device frame 12 . when a plate 9 d 1 of the developing blade 9 d is closely contacted to the flat ( bearing ) surface 12 i of the developing device frame 12 , the abutting surface 74 c 2 of the positioning portion 74 c is pressed by the plate 9 d 1 of the developing blade 9 as shown in ( c ) of fig5 whereby the bent engage portion 74 c 1 of the positioning portion 74 c of the magnetic seal member 71 is pressed into the groove 72 d . this rotates the upper portion of the magnetic seal member 71 about the lower end surface 71 f , so that the elastic lining 77 is compressed rearwardly . the reaction force thereof is received by the lower end abutting portion 72 f of the groove 72 of the developing device frame 12 to which the lower end surface 71 f of the magnetic seal member 71 is contacted and by the plate 9 d 1 to which abutting surface 74 c 2 of the magnetic plate 74 is contacted . then , a developer roller unit g is mounted . this is shown in ( d ) of fig5 . as a result , the magnetic seal member 71 is held by the mounting groove 72 ( for mounting the magnetic seal member 71 ) provided in the developing device frame 12 and the abutting surface 74 c 2 of the magnetic plate 74 is pressed by the plate 9 d 1 of the developing blade 9 d , thus being correctly positioned . as described above , in this embodiment , the magnetic seal member 71 is formed of the magnetic plate 74 , not the magnet 73 at the portion other than the portion ( the arcuate portion ) to be opposite to the peripheral surface of the developing sleeve 5 , so that the amount of use of expensive magnet can be decreased to cut down on costs . further , in this embodiment , a part of ( the magnetic plate of ) the magnetic seal member , i . e ., the portion other than the portion opposite to the developing sleeve 5 , is designed to be elastically deformable , thus improving the assembling performance . in addition , a probability of breakage of the magnetic seal member by the load applied at the time of the assembling is smaller than the case of the conventional magnetic seal member . incidentally , in this embodiment , the magnetic plate 74 located or the outer side of the magnet 73 is integral with the magnetic plate of the positioning portion since a single magnetic plate is subjected to bending to provide such magnetic plate portions . however , the magnetic plate 74 may be one which is not integral with the magnetic plate of the positioning portion . in the above embodiment , as the process cartridge detachably mountable to the main assembly of the image forming apparatus , the process cartridge including the photosensitive drum and process means , acting on the photosensitive drum , which includes the charging means , the developing means and the cleaning means , integrally supported together with the photosensitive drum , is described but it is possible to employ a process cartridge including , in addition to the photosensitive drum and the developing means , either one of the charging means and the cleaning means . further , in the above embodiment , the process cartridge including the photosensitive drum , detachably mountable to the image forming apparatus main assembly but it is possible to independently mounting the respective constitutional components ( means ) to the image forming apparatus or such constitutional components may be those independently detachably mountable to the image forming apparatus . further , the image forming apparatus is the printer in the above embodiment but is also applicable to other image forming apparatus such as a copying machine , a facsimile apparatus and multiple function processing machine including functions of the copying machine and the facsimile apparatus . furthermore , the present invention is not limited to the above described embodiments , and variations and modifications may be made within the scope of the present invention .