PURPOSE: To reduce a controlling section in size and weight, to improve its reliability, and to lower its price by a method wherein functions related with a host interface, print data control, timer, test print function, transmission and reception by microcomputers, etc., are arranged in a gate array.

CONSTITUTION: A CPU checks what is in a printer with the intermediary of a mechanism control interface 3. The CPU, upon detecting no abnormality, informs a host that printing may be performed, and then controls data reception after determining whether print data forwarded by the host to a host interface 2 are parallel or serial. The CPU also determines with help of the mechanism control interface 3 whether the received data if serial is synchronous or asynchronous. Received serial print data turns off or on a laser beam, etc. Simultaneously, the laser beam, etc., is caused to scan a line. The CPU, when a line has been printed out, feeds paper forward, controlling drums and heaters. Paper is expelled for the completion of the printing process.