Abstract: Nuclear magnetic resonance diagnostic apparatus which forms an image of a slice of an object, including an apparatus for detecting NMR signals induced from an object with a condenser and a resonance circuit being provided with a receiving coil which surrounds the object, peakhold means for receiving the NMR signals generated from the resonance circuit to retain the maximum value in the NMR signals in the predetermined order which are picked up in every excitation cycle of the spins, and means for controlling the selection of the NMR signals when the retained value by the peak-hold means is in the synchronous level set previously, utilizing the variation of the NMR signal level in accordance with the variation of floating capacitance of the resonance circuit, which is altered according to the living motion of the object, to detect the state of the living motion, while thereby selecting the NMR signals in the predetermined state to utilize them into image reconstruction.