PURPOSE: To optimize an exposure with respect to a picture input device at a high speed by calculating a proper shutter speed from plural reflection level signals obtained through a changed shutter speed.

CONSTITUTION: A reflected luminous quantity detection section 7 detects a signal whose level is highest among image pickup signals P1 fed from a picture input device 3 as a reflecting level signal P6. A shutter speed calculation section 8 changes the shutter speed of the device 3 through a shutter speed control section 9 to obtain a proportional coefficient between plural obtained signals P6 and the shutter speed thereby calculating an optimum shutter speed based on the proportional coefficient and the desired signal P6. In this case, since a radiation quantity of the lighting means 2 having a slow response speed is constant, the exposure is optimized at a high speed.