PURPOSE:To obtain a nuclear magnetic resonance device which can be operated easily, by controlling a variable capacitor in accordance with a matching deciding signal of an output impedance of a high-frequency power source and an impedance of a high frequency coil system, and adjusting automatically the impedance matching. CONSTITUTION:In a state that an object to be measured has been inserted into a high-frequency coil system, power is supplied to a nuclear magnetic resonance system of a coil 6 and a variable capacitor 2 from a high frequency power source 7. In a directional coupler 10 of an impedance matching deciding means 8, a voltage reflected wave from the resonance system is separated and inputted, and VSWE is measured. This VSWR is supplied to a microcomputer 12 through an A/D converter 11, compared with a reference value stored in a memory 15, and in the even of mismatching, the capacitor 2 is brought to a feedback control through a driving device 17 by a CPU14, an outputting circuit 16, etc., the impedance matching is controlled automatically, and a nuclear magnetic resonance device which can be operated easily is obtained.