PROBLEM TO BE SOLVED: To provide an image processor capable of improving positioning accuracy and shortening a time necessary for positioning, and to provide a therapy system and an image processing method.SOLUTION: An image processor of an embodiment includes a point acquisition part, a point detection part and a calculation part. The point detection part detects a correspondence point corresponding to a set point determined in a plurality of first transparent images obtained by photographing or a plurality of second transparent images reconfigured from volume data in a first transparent image or a second transparent image. The calculation part calculates a positional displacement amount of an object when generating a second transparent image to the position of the object when photographing the first transparent image on the basis of the set point and the correspondence point. If the displacement amount does not satisfy a prescribed condition, the point detection part detects correspondence points in a second transparent image and a first transparent image reconfigured again from volume data moved on the basis of the displacement amount, and the calculation part re-calculates a displacement amount on the basis of a newly detected correspondence point and set point.
