The distance between the radiation source and an object (SOD value) is acquired, distance dependent information which is obtained from a radiation image and changes with the distance between a radiation source and a radiation detector (SID value) is acquired, a temporary thickness of the object is determined by a first function representing the correspondence relationship of first information having at least one piece of the distance dependent information, the SID value, and the thickness of the object, and the temporary SID value is determined by adding the SOD value to the determined value. The thickness of the object is determined by a second function representing the correspondence relationship of second information having at least one piece of the distance dependent information, the SID value, and the thickness of the object, and the SID value is determined by adding the SOD value to the determined value.
