PURPOSE: To provide real-time feedback to a scanning system so as to position successive scanning lines by providing a member which transmits light across a search direction and a surface which partitions an optical pattern along part of the member and does not transmit the crossing light.
CONSTITUTION: An optical pattern area 32 has an enable part 50 and a periodic pattern 52 is present on a side adjacent to the main part of an optical element 30. Therefore, when an irradiation point like a beam spot passes through the synchronizing pattern 52, the input beam causes repetitive optical modulation. Once a detection part 42 detects this periodic pattern, its electric signal output becomes a repetition signal. The border between the enable part 50 and a period part 52 is detected by the detector 42 and a control system uses it to set the start of a scanning line; and the period pattern 52 itself is used to synchronize a pixel clock to an accurate phase for controlling the timing of data to a laser source.