A radiation imaging apparatus (10) is provided with a differential phase image producing section (40), a phase unwrapping section (41), a statistical operation section (44), and a correction processing section (45). The differential phase image producing section produces a differential phase image (61) in which pixel values are wrapped into a predetermined range α, based on image data obtained with an X-ray image detector (13). The phase unwrapping section performs a phase unwrapping process to the differential phase image. The statistical operation section obtains a mode from statistical operation of pixel values in each subregion (71, 71a to 71c, 71k) segmented in the unwrapped differential phase image. Each subregion is a unit in which error caused by the phase unwrapping process is to be corrected. The correction processing section calculates, for each pixel (30), an integer "n" which allows a difference Δ between the mode and a pixel value of each pixel to satisfy nα-α/2≤Δ
