PURPOSE: To precisely detect the timing with which reflected slit light passes through a cell constituting an image-pickup plane.

CONSTITUTION: Two photosensors 20A, 20B are arranged adjacent together in a cell, along the travel direction of reflected slit light, and photocurrents delivered from these photosensors are detected by current mirror circuits 22A, 22B. Further, a current mirror circuit 22C detects a difference between outputs from the circuits 22A, 22B, and invetors 24A to 24C binarize the difference. The timing with which the binary output is turned into a H-level from a L-level is determined as the timing of passing of the reflected slit light through the cell.