An engine control section detects that a sheet in a cassette is exhausted during duplex image formation. If a sheet with its first surface being printed is on standby in a duplexer, an image formation timing signal is output for a first surface of a sheet not fed because of a paper-out. Subsequently, an image formation timing signal is output for a second surface of the sheet that is in the duplexer and has its first surface printed. After completing the image formation of the second surface of the sheet, the operation is halted.