To provide an NMR locking device hardly affected by noises even when an S/N ratio of an NMR signal coming from a locking sample is bad.

A means is provided for determining a low frequency thermal noise level of a preamplifier 7 when a switch 3 is not connected to a high frequency oscillator 1 for locking or a transmitting and receiving coil 5. During a period where a lock dispersion signal outputted from the transmitting and receiving coil 5 via the preamplifier 7 and a phase detector 9 into an integrating circuit 12 is cut by a gate 8, a signal of the low frequency thermal noise level determined just before this period is put on hold by a holding circuit 21 for outputting it into the integrating circuit 12 instead of the lock dispersion signal.