1. A medical image diagnosis apparatus, comprising: a storage unit that stores material distribution information, which is acquired from an object including a target organ that moves periodically, and movement information, which is measured together with the material distribution information; an image reconstruction unit that reconstructs a plurality of tomographic images of different movement phases using the material distribution information and the movement information; a superimposed image creation unit that creates a superimposed image by superimposing the plurality of tomographic images; a specific position calculation unit that calculates a specific position of the target organ based on the superimposed image; and a specific phase determination unit that determines a specific phase of the target organ based on the specific position.