In order to provide a medical image diagnosis apparatus and a phase determination method using a medical image diagnosis apparatus that can determine an appropriate movement phase when obtaining a tomographic image corresponding to a specific movement phase, there are provided 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.