In order to provide a particle radiotherapy apparatus with high sensitivity for detecting annihilation radiation pairs, an elliptic detector ring of a particle radiotherapy apparatus according to this invention makes rotating movement relative to a top board. Specifically, with rotation about a base axis of a first ring and a second ring, the elliptic detector ring makes rotating movement in a state of being tilted relative to the first ring. Incidentally, the elliptic detector ring cannot be disposed in a position to interfere with travel of this particle beam. According to the construction of this invention, the elliptic detector ring is tilted relative to the top board, and besides makes rotating movement relative to the top board. Since the elliptic detector ring can be moved away from the particle beam by rotating the elliptic detector ring, it is possible to provide the particle radiotherapy apparatus which can detect annihilation radiation while emitting the particle beam.