To provide a radiation image detector, a radiation imaging device and a radiation imaging system that use a stably producible grating to obtain a phase contrast image having stable image quality.

A radiation imaging system comprises: a first grating 31 having a plurality of strip bands arranged in a first direction (x direction); a second grating 32 having a plurality of strip bands arranged in a period substantially coinciding with a pattern period of a radiological image to be formed by a radiation having passed through the first grating 31; and a radiation image detector 30 using a plurality of pixels 40 to detect the radiological image masked by the second grating 32. The first and second gratings 31 and 32 include a plurality of grating pieces 311 and 321 that are arranged at least in the first direction (x direction) in a plane intersecting with an optical axis of the radiation, respectively, and an array pitch P_C of the grating pieces 311 and 321 in the x direction is twice or more a pixel pitch P_D in the x direction of the radiation image detector 30.