A radiation image processing apparatus which processes non-spiral projection data obtained by radiation imaging performed by rotating a radiation generator and a two-dimensional radiation detector relative to a subject reconstructs the non-spiral projection data into first volume data subjected to an averaging process in a direction of a rotation axis of the rotation and second volume data not subjected to the averaging process. The radiation image processing apparatus performs planar reconstruction calculation using the first volume data to generate a cross section image in a first plane perpendicular to the direction of the rotation axis and performs planar reconstruction calculation using the second volume data to generate a cross section image in a second plane parallel to the direction of the rotation axis. The resulting cross section images are displayed on a display apparatus.