PURPOSE: To automatically adjust focus by controlling an optical system lens transfer mechanism to minimize the difference of a picture signal in width in two threshold levels of an image pickup output.

CONSTITUTION: An electric signal from a photoelectric conversion element 3 is converted into digital data by an A/D converter 8, fetched in an arithmetic processing circuit 12 to calculate average values, etc. The average values thus obtained are set in a threshold value register 10; the binary data which is banary- coded by using the average values as threshold values into are obtained in the output of a digital comparator 9; and a histogram of one line is obtained by a histogram register 11 at the next stage. When scanning, this focus adjusting device sets threshold values greater than the average values and those smaller than the average values in the register 10 to obtain histograms W1 and W2. The device calculates the difference between W1 and W2 by the circuit 12, and controls a lens transfer mechanism 16 to minimize the difference.