A gamma camera system is provided for obtaining a distribution image (i.e. a reconstruction and/or multiplanar reconstruction image) of a radioisotope injected into an object by dynamic study on the basis of .gamma.-rays emitted from the radioisotope. The system comprises a two-dimensional detector for detecting the .gamma.-rays, the detector being repeatedly rotated around the object. There are provided an element for forming image data of a reference image and the distribution image on the basis of detected signals from the detector, an element for obtaining a change in quantity of the .gamma.-rays every one rotation of detector, an element for specifying one of the plurality of rotations, the specified rotation corresponding to a maximum number of the .gamma.-rays, and an element for displaying the reference image using the reference image data corresponding to the specified one rotation. Further provided is an element for manually setting information required to obtain the distribution image using the displayed reference image. Using manually set information, the image data forming element creates the distribution image data.