Projection data are obtained by using a method for imaging by rotating the first imaging system having an X-ray generating unit 1a and the second imaging system having an X-ray generating unit 1b. The obtained projection data are subjected to a reconstruction process to generate X-ray image data. In this case, a relative angle .eta.0 formed between the imaging systems is setup according to a palmic period T0 of a subject and a rotation velocity Vr of the imaging systems. Thus, the projection data from more directions at a predetermined palmic time-phase are collected without overlapping.