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