1. An X-ray CT apparatus which performs image reconstruction on the basis of parallel beam data obtained by fan-parallel conversion on fan beam data, the X-ray CT apparatus comprising: a determination unit which sets parallel beam data having a phase to be processed as target data, sets parallel beam data having a phase opposing target data as opposing data, and determines the presence/absence of opposing data corresponding to target data; and an image reconstruction unit which performs different high-resolution reconstruction depending on the determination result of the determination unit, executes a back projection process on the basis of the distance between a pixel boundary and a beam boundary using target data or target data and opposing data, and performs image reconstruction.