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.