PURPOSE: To enable smiple and accurate focusing by setting two, high and low slice levels different in voltage, subtracting the number of pulses of slice level of high voltage from the number of pulses of slice level of low voltage in video output signals, and adjusting to make the numerical value minimum.

CONSTITUTION: When an object is in focus, the video output signal of an image sensor give a large difference in voltage between an input light pulse and an adjoining lighe sheltering pulse at both end parts of a picture, and when the object is out of focus, the difference in voltage between an input light pulse and an adjoining sheltering pulse is small, and the difference widens slowly. Basing on this principle, tow, high and low, slice levels 6h, 6l different in voltage are set to video output signals 5 of the image sensor 1, and at the same time, the number of pulses of high voltage slice level 6h is subtracted from the number of pulses of low voltage slice level 6l through a digit indicator 4 and the minimum value (normally 1 or 0) is determined.