A method and apparatus for digital quadrature lock-in detection capable of receiving magnetic resonance signals or electron spin resonance signals at high sensitivity. The method starts with digitizing a signal wave consisting of a magnetic resonance or electron spin resonance signal. The digitized signal wave is multiplied by digitized reference waves of sine and cosine functions to obtain signals of real and imaginary parts which are 90.degree. out of phase (multiplying step). The frequencies of the sine and cosine functions are varied according to the observation width. The multiplying step is repeated.