An X-ray CT device (10) according to the present invention produces an X-ray beam of a cone shape in the direction of the body axis of a subject (P) from an X-ray source (14), performs a helical scanning operation in accordance with the relative movement and relative rotational motion of the X-ray source (14) and the subject (P), processes data that is collected by a data collecting unit (46), and reconstructs an image by back-projecting the processed data at a reconstruction processing unit (48). When the moving speed of the bed (18) is defined, a system controlling unit (40) determines the optimal collection condition for a two-dimensional array-type X-ray detector (16).
