1. A radiographic phase-contrast imaging apparatus comprising: a radiation source; a first grating having a periodically arranged grating structure and allowing radiation emitted from the radiation source to pass therethrough to form a periodic pattern image; a second grating having a periodically arranged grating structure including areas transmitting the periodic pattern image formed by the first grating and areas shielding the periodic pattern image; a radiographic image detector including two-dimensionally arranged pixels for detecting the radiation transmitted through the second grating, wherein the first grating and the second grating are adapted to form a moire pattern when the periodic pattern image formed by the first grating and the second grating are superimposed one another; and a phase-contrast image generating unit for obtaining image signals of a plurality of fringe images based on an image signal of the moire pattern detected by the radiographic image detector, and generating a phase-contrast image based on the obtained image signals of the fringe images, wherein the fringe images correspond to different pixel groups located at different positions from one another with respect to a predetermined direction, each pixels group includes pixels arranged at intervals of a predetermined number of pixels in the predetermined direction, image signals read out from the pixels of each pixel group are obtained as the image signal of each fringe image, and the predetermined direction is a direction parallel to or a direction intersecting a period direction of the moire pattern other than a direction orthogonal to the period direction of the moire pattern.