To secure appropriate exposure in accordance with the luminance of a subject being a processed object and to improve accuracy in image processing by controlling the shutter speed of a camera so that section length on a low luminance side to the 1st index value of an integrated value obtained by adding the frequency of the histogram of luminance distribution may be equal to the section length on a high luminance side to a 2nd index value.

The histogram for controlling a shutter is formed by dividing the luminance distribution in a sample image into every specified luminance gradation, allotting the index value of i=0 to (n) to each section and setting the index value as the axis of abscissa and the frequency of each index value as the axis of ordinate. By using the section length D' to the section index value D on the low luminance side of the historgram and the section length B' to the section index value B on the high luminance side, a shutter control speed value (t) is found by t=t0+K.(D'-B') assuming that the present shutter speed control value is t0 and a control gain is K, and the shutter speed is controlled so that the section length D' may be equal to the section length B'.