According to the present invention, a subject is prompted to take breaths that are different at least in depth, and the subject's motion associated with respiration is captured. A timing when the subject is scanned is controlled according to the captured motion of the subject associated with respiration. A two-dimensional detector formed like a two-dimensional plane detects an X-ray beam having passed through the subject. Data is collected from the two-dimensional detector to acquire volume data on the subject.