A medium area (S) filled with liquid xenon (2) is formed between an external cylindrical body (1a) and internal cylindrical body (1b), and a pair of anode pads (11, 12) are disposed in two-dimensional form in opposite end portions of the medium area (S) in the intersection direction with respect to the gamma-ray incident direction. An intermediate electrode (10) is disposed between a pair of anode pads (11, 12), and a plurality of photomultiplier tubes (5) is installed in two-dimensional form in the external cylindrical body (1a). Then, the gamma-ray reaction point within the liquid area (S) is identified from signals output from the anode pads (11, 12) and photomultiplier tubes (5).