PURPOSE: To rapidly perform magnetic field generating processing while reducing the load of a computer, by preliminarily calculating the coefficient data for calculating the components in three reference axial directions crossing at right angle to each other of gradient magnetic field data in a projection direction and an orthogonal direction prior to measurement.

CONSTITUTION: Prior to starting measurement, the coefficient data for calculating components in three reference axial directions crossing at right angles to each other of the gradient magnetic field data in the direction at right angles to a projection direction are calculated by the control computer 4 of a control part 3 to be held to a coefficient register part 14. The gradient magnetic field data in each axial direction is calculated by the operation of a multiplier part 15 and an adder part 16 on the basis of the gradient magnetic field data in the projection direction and the orthogonal direction read from a wave form memory part 13 and each coefficient data of the register part 14 during the practice of measurement. On the basis of the calculated gradient magnetic field data in each axial direction, desired gradient magnetic fields in the projection and the orthogonal direction are generated.