A LAN control unit receives print data from a client device or the like. An input job storage unit registered on a hard disk a series of PDL commands included in the print data received. An input job queue management unit registers print job specifying information specifying a print job represented by the print data received to the end of an input job queue. A PDL interpretation/execution unit successively executes from the head of the series of PDL commands stored on the hard disk device. When it is determined that the PDL command that has been executed is a re-execution unnecessary command, the PDL interpretation/execution unit overwrites the PDL command stored on the hard disk with a NOP command.