1. A gamma camera apparatus for detecting gamma rays emitted from a radioisotope administered to an examinee to construct a three-dimensional image representing a distribution of the radioisotope in the examinee, comprising: detecting means for detecting gamma rays emitted from the radioisotope in the examinee; an encoding aperture plate disposed between the examinee and said detecting means; adjusting means for adjusting the distance from said detecting means to said encoding aperture plate to adjust the position of said encoding aperture plate depending on the depth of an observation position in the examinee; and processing means for reconstructing a three-dimensional image based on the gamma rays emitted from the radioisotope and detected by said detecting means through said encoding aperture plate.