1. A movable part tracking and treatment apparatus, comprising: an acquisition unit; a first projection image generation unit; an affected part image extraction unit; a tracing target image extraction unit; a first parameter derivation unit; and a second parameter derivation unit, wherein the acquisition unit is adapted to acquire a three-dimensional moving image by imaging an inside of a body of a patient; the first projection image generation unit is adapted to generate a first projection moving image by projecting the three-dimensional moving image on a two-dimensional surface from a fixed direction, the three-dimensional moving image including a tracing target and an affected part area that are part of internal organs in a displaced state, the affected part image extraction unit is adapted to extract the displaced affected part area from the first projection moving image, the tracing target image extraction unit is adapted to extract the displaced tracing target from the first projection moving image, the first parameter derivation unit is adapted to derive a first parameter indicative of position information on a beam irradiation point selected from the displaced affected part area in the first projection moving image, and the second parameter derivation unit is adapted to derive a second parameter necessary to extract the corresponding tracing target from another projection moving image based on the tracing target extracted from the first projection moving image.