PURPOSE: To perform an optimum output operation by adjusting competing data inputs from plural data generation sources.

CONSTITUTION: For example, when there are data received from another host through a centronics interface part 4 while, for example, an LBP 100 received data from a host through an RS-232C interface part 3 and performs image processing and output operation, the image processing and output operation based upon the data from the host are centinued until one of events of (1) recognition of 'EOF', (2) detection of 'time-out', (3) 'end of one job prescribed by PDL'. (4) detction of the 'out page quantity' over state, etc., for which a user preuiously set is generated. After the even is generated, image processing and output operation based upon the data from the other host are performed.