An image forming apparatus includes: a LAN I/F for communication with external devices through a network; a printer engine for executing printing; an HDD for storing file name and transmission source information of printed data; a CPU executing, if the LAN I/F receives a print instruction of data having the same file name and same transmission source as data stored in the HDD, a program for detecting wasteful printing, based on match rate of the received data with respect to the printed data and on accumulated number of printed copies related to the transmission source; and an engine I/F for transmitting data and the like to the printer engine, under the control of CPU.