Two detector units and two holding arms are provided. The detector units are each disposed on a part of an arc of a circle surrounding a subject. The holding arms each disposed on the arc hold the detector units individually. The holding arms move independently along the circle surrounding the subject, thereby causing the detector units to move. Consequently, a first detector unit is held with a first holding arm entirely, and a second detector unit is held with a second holding arm entirely. This causes less swing of the detector units achieving a simple configuration.
