a best mode for carrying out the invention will hereinafter be explained in detail with reference to the figures . incidentally , the present invention is not limited to the best mode for carrying out the invention . a block diagram of an mri apparatus is shown in fig1 . the present apparatus is one example of the best mode for carrying out the invention . one example of the best mode for carrying out the invention related to the mri apparatus is shown according to the construction of the present apparatus . as shown in fig1 , the present apparatus has a magnetic field generator 100 . the magnetic field generator 100 has main magnetic field magnet units 102 , gradient coil units 106 and rf ( radio frequency ) coil units 108 . any of the main magnetic field magnet units 102 , gradient coil units 106 and rf coil units 108 comprises paired ones opposed to one another with a space interposed therebetween . further , any of them has a substantially disc shape and is placed with its central axis held in common . each of the gradient coil units 106 is one example of a gradient magnetic field coil employed in the invention . each of the rf coil unit 108 is one example of an rf coil employed in the invention . a target 1 is placed on a table 500 in an internal bore of the magnetic field generator 100 and carried in and out . the table 500 is driven by a table driver 120 . each of the main magnetic field magnet units 102 forms a static magnetic field in the internal bore of the magnetic field generator 100 . the direction of the static magnetic field is approximately orthogonal to the direction of a body axis of the target 1 . that is , each of the main magnetic field magnet units 102 forms a so - called vertical magnetic field . each of the main magnetic field magnet units 102 is configured using a permanent magnet . the gradient coil units 106 produce three gradient magnetic fields for respectively causing the intensities of static magnetic fields to have gradients or slopes in the directions of three axes vertical to one another , i . e ., a slice axis , a phase axis and a frequency axis . each of the gradient coil units 106 has unillustrated 3 - systematic gradient coils in association with the three gradient magnetic fields . each of the rf coil units 108 transmits an rf pulse ( radio frequency pulse ) for exciting a spin in a body of the target 1 to a static magnetic field space . further , the rf coil unit 108 receives therein a magnetic resonance signal which produces the excited spin . the rf coil units 108 may perform transmission and reception by either the same coil or discrete coils . a gradient driver 130 is connected to the gradient coil units 106 . the gradient driver 130 supplies a drive signal to each of the gradient coil units 106 to generate a gradient magnetic field . the gradient driver 130 has unillustrated 3 - systematic drive circuits in association with the 3 - systematic gradient coils in the gradient coil unit 106 . an rf driver 140 is connected to the rf coil units 108 . the rf driver 140 supplies a drive signal to each of the rf coil units 108 to transmit an rf pulse , thereby exciting the spin in the body of the target 1 . a data acquisition unit 150 is connected to each of the rf coil units 108 . the data acquisition unit 150 takes in or captures signals received by the rf coil units 108 by sampling and collects or acquires the same as digital data . a controller 160 is connected to the table driver 120 , the gradient driver 130 , the rf driver 140 and the data acquisition unit 150 . the controller 160 controls the table driver 120 to data acquisition unit 150 respectively to execute shooting or imaging . the controller 160 is configured using a computer or the like , for example . the controller 160 has a memory . the memory stores a program and various data for the controller 160 therein . the function of the controller 160 is implemented by allowing the computer to execute the program stored in the memory . the output side of the data acquisition unit 150 is connected to a data processor 170 . the data acquired by the data acquisition unit 150 is inputted to the data processor 170 . the data processor 170 is configured using a computer or the like , for example . the data processor 170 has a memory . the memory stores a program and various data for the data processor 170 therein . the data processor 170 is connected to the controller 160 . the data processor 170 ranks ahead of the controller 160 and generally controls it . the function of the present apparatus is implemented by allowing the data processor 170 to execute the program stored in the memory . the data processor 170 causes the memory to store the data captured by the data acquisition unit 150 . a data space is defined in the memory . the data space forms a fourier space . the fourier space is also called “ k - space ”. the data processor 170 transforms data in the k - space into inverse fourier form to thereby reconstruct an image for the target 1 . a display unit 180 and an operation or control unit 190 are connected to the data processor 170 . the display unit 180 comprises a graphic display or the like . the operation unit 190 comprises a keyboard or the like provided with a pointing device . the display unit 180 displays a reconstructed image and various information outputted from the data processor 170 . the operation unit 190 is operated by an operator and inputs various commands and information or the like to the data processor 170 . the operator is able to control the present apparatus on an interactive basis through the display unit 180 and the operation unit 190 . an outer appearance of one example of the magnetic field generator 100 is shown in fig2 by a perspective view . the magnetic field generator 100 is one example of the best mode for carrying out the invention . one example of the best mode for carrying out the invention related to the magnetic field generator is shown according to the construction of the magnetic field generator 100 . the magnetic field generator 100 is also one example of a magnetic field generator according to the invention . as shown in fig2 , the magnetic field generator 100 includes a pair of main magnetic field magnet units 102 supported by a yoke 200 . each of the main magnetic field magnet units 102 is one example of a permanent magnet employed in the invention . the yoke 200 is one example of a yoke employed in the invention . the main magnetic field magnet units 102 respectively have substantially disc - shaped or short cylindrical outer shapes . the yoke 200 forms return passes used for the pair of main magnetic field magnet units 102 and is constituted of a ferromagnetic material such as soft iron in a substantially c form . incidentally , the shape of the yoke 200 is not limited to the c form . the pair of main magnetic field magnet units 102 is supported in parallel and coaxially in such a manner that magnetic poles opposite in polarity to each other are opposed to each other . thus , a vertical magnetic field is formed between both magnetic poles . the direction of the magnetic field is assumed to be a z direction below . while the pair of gradient coil units 106 and the pair of rf coil units 108 are respectively provided over magnetic pole faces of the pair of main magnetic field magnet units 102 , their illustrations are omitted . each of the main magnetic field magnet units 102 comprises a permanent magnet 122 and a pole piece 124 . the permanent magnet 122 is a magnet composed of an nd — fe — b alloy , i . e ., a neodymium magnet . the neodymium magnet does not contain a rare element like dysprosium and is a magnet relatively small in hcj . the pole piece 124 is made up of soft iron . the diameter of the pole piece 124 is smaller than the diameter of the magnetic pole face of the main magnetic field magnet unit 102 . that is , the permanent magnet 122 has a pole piece smaller in area than its magnetic pole face . bh curves of the permanent magnet 122 are shown in fig3 . as shown in fig3 , the bh curve is given by a linear curve l 1 at a temperature t 1 , whereas the bh curve results in a combination of a linear curve l 2 parallel - moved downwardly and a non - linear curve l 3 at a temperature t 2 (& gt ; t 1 ). an operating point p of the permanent magnet 122 is placed on the non - linear curve l 3 . thus , an operating bh curve at the temperature t 2 results in a linear curve l 4 parallel - moved further down as indicated by a broken line l 4 . returning the temperature to ti from this state yields the bh curve as a broken line l 1 ′. the broken line l 1 ′ is parallel to the linear curve l 1 but becomes a linear curve displaced in position . when , however , the temperature is raised to t 2 again and is returned to t 1 , the bh curve is restored to the broken line l 1 ′. this is the same no matter how many times it is repeated . that is , after a temperature history of t 1 - t 2 - t 1 is passed one cycle , the bh curve at the temperature t 1 reaches the broken line l 1 ′ no matter how many times the same cycle is repeated . thus , once the temperature history of t 1 - t 2 - t 1 is made , the temperature characteristic of the permanent magnet 122 assumes reversibility . utilizing this phenomenon enables reversing of the temperature characteristic of the magnetic field generator using the permanent magnets small in hcj . a process for reversing the temperature characteristic is shown in fig4 . the present process is one example of the best mode for carrying out the invention . one example of the best mode for carrying out the invention related to a thermal controlling method is shown based on the present process . as shown in fig4 , the temperature is raised from room temperature to a temperature higher than it at step s 401 . at step s 402 , the temperature higher than the room temperature is maintained . at step s 403 , the temperature is lowered from the temperature higher than the room temperature to the room temperature . the room temperature is equivalent to the temperature t 1 , and the temperature higher than the room temperature corresponds to the temperature t 2 . incidentally , the room temperature ranges from 10 ° c . to 25 ° c ., for example . step s 401 is one example of a temperature rise step according to the invention . step s 402 is one example of a maintenance step according to the invention . step s 403 is one example of a temperature fall step according to the invention . one example of a further detailed process of the thermal controlling is shown in fig5 . as shown in fig5 , the temperature is raised from the room temperature to 35 ° c . at step s 501 , the temperature of 35 ° c . is maintained over two hours at step s 502 , the temperature is raised from 35 ° c . to 45 ° c . at step s 503 , the temperature of 45 ° c . is maintained over two hours at step s 504 , the temperature is lowered from 45 ° c . to 35 ° c . at step s 505 , the temperature of 35 ° c . is maintained over 1 hour at step s 506 , and the temperature is lowered from 35 ° c . to the room temperature at step s 507 . step s 501 is one example of a first temperature rise step according to the invention . step s 502 is one example of a first maintenance step according to the invention . step s 503 is one example of a second temperature rise step according to the invention . step s 504 is one example of a second maintenance step according to the invention . step s 505 is one example of a first temperature fall step according to the invention . step s 506 is one example of a third maintenance step according to the invention . step s 507 is one example of a second temperature fall step according to the invention . with such thermal controlling , the temperature characteristic of the magnetic field generator is reliably reversed . incidentally , the thermal operation or controlling of the magnetic field generator may be performed before degree - of - uniformity control on a magnetic filed strength distribution , i . e ., its shimming . such thermal controlling may be carried out before shipment of a product when a maker executes it , whereas when a user performs it , such thermal controlling may be carried out upon its acceptance . a magnetic flux density distribution and a magnetic field strength distribution at a magnetic pole face of each permanent magnet 122 are respectively shown in fig6 ( a ) and 6 ( b ). as shown in fig6 ( a ) and 6 ( b ), the peripheral edge portion of the permanent magnet 122 becomes lower than a portion located inside of it in magnetic flux density and magnetic filed strength . a portion low in magnetic flux density and magnetic field strength is equivalent to a portion low in operating point , i . e ., a portion large in demagnetization . a portion high in magnetic flux density and magnetic field strength is equivalent to a portion high in operating point , i . e ., a portion low in demagnetization . in view of such a magnetic state , the peripheral edge portion 122 b is constituted of a magnet large in hcj and the portion 122 a located inside of it is constituted of a magnet small in hcj , as shown in fig7 . by doing so , any operating point can be set so as to fall within a linear region of a bh curve . this is because since the linear region of the bh curve is large although the operating point is low at the peripheral edge portion 122 b large in hcj , the operating point thereof exists in the linear region , whereas since the operating point is high at the inner portion 122 a low in hcj , the operating point thereof exists in the linear region even though the linear region of the bh curve is small . thus , the magnetic field generator 100 has a reversible temperature characteristic . since , at this time , the most part of the permanent magnet 122 is made up of the magnet large in hcj and only the peripheral edge portion is made up of the magnet large in hcj , the permanent magnet 122 results in one much more inexpensive than when the whole part thereof is constituted of the magnet large in hcj . many widely different embodiments of the invention may be configured without departing from the spirit and the scope of the present invention . it should be understood that the present invention is not limited to the specific embodiments described in the specification , except as defined in the appended claims .