In a particle radiotherapy apparatus which has a passage for allowing movement of a particle beam, this invention provides a particle radiotherapy apparatus with high sensitivity for detection of annihilation radiation pairs even if there is a difference between a point where the particle beam loses energy and a position of a detector ring in a body axis direction of a patient. For the purpose of solving such a problem, the particle radiotherapy apparatus according to this invention includes an elliptic detector ring which is vertically long and is reversibly tiltable. Consequently, annihilation gamma ray pairs are detected with the single elliptic detector ring. Then, annihilation gamma ray pairs occurring inside the elliptic detector ring will be detected. In other words, all annihilation gamma ray pairs will impinge on the single elliptic detector ring at substantially right angles. This inhibits lowering of sensitivity for detection.