PURPOSE: To discriminate easily the propriety of focusing of an image pickup device by detecting a slow change of a boundary between a bright and a dark part by two comparators and counting the change while converting it into a pulse.

CONSTITUTION: A video pulse signal having a level voltage in response to an input light amount is generated from a linear solid-state image pickup element in scanning a dark object 1 with a bright background. The change is steep in the boundary between the object and the background when a lens 2 is focused and slow when the focus is not matched. In setting reference voltages E1, E2 of comparators 6, 7 to 2/3, 1/3 of the voltage of a bright image, a pulse corresponding to a bright part, an intermediate part and a dark part is obtained respectively from AND gates 81, 82, 83. In setting a suitable amount to P1, P2 to discriminate dark and bright, an AND gate 12 is opened in a suitable timing, the number of pulses representing the intermediate brightness is displayed on a display device 13, and it is discriminated that the focus is matched when the value is small.