An NMR apparatus provided with a lock device having improved monotonicity without reducing a control range. In a lock device including a lock transmitter, a lock receiver, a lock detector, a lock corrector, a lock power supply and a lock coil, the lock detector performs complex detection of a detection signal and outputs to the lock corrector an absorption signal SA which is a real number component of the detection signal and a dispersion signal SD which is an imaginary number component of the detection signal; the lock corrector obtains a value SC by multiplying together SD/(SD.sup.2+SA.sup.2) and a value G, and the lock power supply performs adjustment so that a current flowing through the lock coil is proportional to SC. Different constants G are selectively used according to the value of SD/(SD.sup.2+SA.sup.2).