A distance between a radiation source standard point indicating a position of a radiation source and a photographic subject on a standard line passing through the radiation source standard point and a detector standard point indicating a position of a detector is measured, a distance between a first reference point positioned in a first direction which is directed towards the detector standard point from the radiation source standard point with respect to the detector standard point and the detector standard point is measured, a distance between a second reference point positioned in a direction opposite to the first direction with respect to the radiation source and the radiation source standard point is measured, and a subject thickness of the photographic subject is calculated by using the distances and a distance from the distance from the first reference point to the second reference point.
