A pulse pattern forming a pulse sequence is repeatedly generated according to a repetition frequency Fp, and an original pulse train is generated. A modulation signal which repeats ON and OFF states at a modulation frequency Fm is generated. Fp and Fm are in a relationship Fp=2n×Fm, wherein n is an integer greater than or equal to 1. The original pulse train is modulated according to the modulation frequency Fm, and a pulse train signal is generated. A microwave signal is modulated by the pulse train signal and is supplied to an electron spin resonance chamber. By executing a lock-in demodulation on a detection signal reflecting electron spin resonance using the modulation frequency Fm, an ESR signal is obtained.