PURPOSE: To enable readily realizing drive timing control of respective light emitting devices arranged in a matrix pattern and miniaturizing and simplifying a control system.

CONSTITUTION: Exposure data ID for eighty beams are caused by a light address counter 14 to be sequentially written into line memories of control units 100 to 1000 allotted for corresponding lines. Reading of the exposure data ID is started on the basis of an outstart signal OTST and a read clock RCK. At this time, read address counters for the respective control units 100 to 1000 start counting with respect to an offset address. As a result, starting of reading of exposure data for a second line is delayed eight clocks relative to that for a first line, and starting of reading of exposure data for a tenth line is delayed seventy-two clocks relative to that for the first line. With respect to respective lines, the read address counters read exposure data ID for eighty beams every one clock to transmit the same to eight row latches. Respective exposure data are added to respective laser diodes in accordance with an exposure position clock ET.