PURPOSE: To prevent white painting-out by adjusting the conversion level based on an output digital signal in an A/D conversion circuit for video signal.

CONSTITUTION: An analog signal Sl is amplified by an amplifier 1 and has the black level adjusted by a clamping circuit 2, and a digital signal S2 is outputted by an A/D converter 3. The signal S1 is set to an all white signal to indicate the start of adjustment from a remote controller 11. A window comparator 4 outputs logical '1' in the range of a certain level of the signal S2, and a counter 5 counts this signal period by a clock signal S3. A control part 7 periodically takes in the counted output value of the counter 5 by a timer 10 and resets the timer. A gain control voltage is outputted to control the gain of the amplifier 1 based on this counted value in accordance with procedures of a level adjustment processing part 12, and the gain control voltage is changed in such direction that the counted value is increased, and an optimum gain control voltage is obtained when the counted value is maximum. The clamping level of the clamping circuit 2 is adjusted in the same manner.