A field gradient generator in an NMR imaging apparatus comprises three sets of field gradient generating coils for respectively generating field gradients in three orthogonal directions in a space to be observed. Each set of coils includes a plurality of unit coils through which individual currents respectively flow. There are prepared a plurality of unit drivers which are for exclusive uses of the respective unit coils or are connected arbitrarily to each unit coil. Thereby, the lowering of an allowable operating voltage is implemented by use of a parallel operation of the unit drivers.