with a view toward implementing a thermal controlling method for making reversible a temperature characteristic of a magnetic field generator using permanent magnets small in hcj , a magnetic field generator whose temperature characteristic is reversible , using permanent magnets small in hcj , and an mri apparatus provided with such a magnetic field generator , there is provided a method for controlling the temperature of a magnetic field generator having a pair of disc - shaped permanent magnets whose magnetic poles opposite in polarity to each other are opposed to each other with spacing defined therebetween , and a yoke that forms return passes for magnetic fluxes of the permanent magnets , comprising the steps of raising the temperature from room temperature to a temperature higher than the room temperature , maintaining the temperature higher than the room temperature , and lowering the temperature from the temperature higher than the room temperature to the room temperature , whereby the temperature characteristics of the permanent magnets are made reversible .