An arithmetic processing unit divides a modified NC program into a plurality of segments and determines a machining time estimation target segment among the plurality of divided segments. When estimating the machining time required for the machining of a machine tool controlled based on the modified NC program, the arithmetic processing unit shortens the time required for the machining time estimation process by reutilizing an estimated machining time for an NC program before modification with respect to segments other than the machining time estimation target segment.