1. A method for imaging nuclear magnetic resonance (NMR) signals from of a target by using non-linear magnetic field gradient, comprising steps of applying a homogeneous static magnetic field to said target; applying a high-frequency (HF) magnetic field in a pulse-like manner from a transmitter coil to said target at right angles to said static magnetic field so as to excite nuclear magnetization in said target, said HF magnetic field having a Larmor frequency corresponding to intensity of said homogeneous static magnetic field; immediately thereafter applying a specifying magnetic field with a non-linear gradient of magnetic field intensity to said target for a certain period of time so as to spatially encode the phase of the nuclear magnetization in a certain locality of the target in a non-linear fashion by causing free precession of said nuclear magnetization, said specifying magnetic field having a magnetic field intensity varying in a non-linear manner; spatially scanning said specifying magnetic field by shifting a center of said non-linear gradient thereof over a certain scan distance by a specifying field scanner; successively repeating said encoding while effecting said spatial scanning of the specifying magnetic field at each encoding; detecting encoded NMR signals from said target by a receiver coil; and computer-processing a plurality of the encoded NMR signals having different degrees of contribution from the nuclear magnetizations in different localities of the target, so as to image information carried by the encoded NMR signals relating to distribution of nuclear magnetic substance in said target.