1. A gamma camera system for obtaining a distribution image of a radioisotope injected into an object being examined by dynamic study on the basis of .gamma.-rays emitted from the radioisotope, the system comprising: a detector for detecting the .gamma.-rays; means for repeatedly providing a plurality of relative rotations between the detector and the object; means for forming image data including data of a reference image and data of the distribution image on the basis of detected signals from the detector; means for obtaining a change in quantity of the .gamma.-rays every one rotation among the plurality of relative rotations between the detector and the object; means for specifying one of the plurality of relative rotations in conjunction with the change in the .gamma.-ray quantity; and means for displaying the reference image using the data of the reference image corresponding to the specified one rotation.