In an NMR apparatus having an analog-to-digital converter for converting the voltage signal V(t) to a digital signal, a reference signal generator for generating reference waves which are digital signals, and a quadrature detector for performing a quadrature detection operation, a detection controller for changing a frequency of .omega.r of the reference waves or a time interval of .DELTA. which is a time interval between data points of digitized NMR signals and also a time interval between data points of the reference waves during detection is installed and furthermore, a storage device for preserving the voltage signal V(t) converted to a digital amount at least until the quadrature detector or the reference signal generator changes the frequency .omega.r of the reference waves or the time interval .DELTA. is installed between the analog-digital converter and the quadrature detector.