1. A X-ray image capturing system, comprising: a X-ray source which emits X-rays; a first grating and a second grating; a subject platform; a X-ray detector in which conversion elements which generate electric signals according to the emitted X-rays are two dimensionally arranged and which reads the electric signals generated by the conversion elements as image signals, and a diagnosis image forming unit which is structured to capture both (a) one or more moire images with a subject placed on a subject platform (b) one or more moire images without the subject being placed on the subject platform, and the diagnosis image forming unit is further structured to form a subject reconstruction image for diagnosis on a basis of both (a) the one or more moire images with the subject, and (b) the one or more moire images without the subject, wherein every time the diagnosis image forming unit performs the image capturing with the subject, the diagnosis image forming unit performs the image capturing without the subject.