PURPOSE: To quickly derive a focused position, and to obtain a clear image by utilizing a contrast between an object and its background, and executing a numerical processing to an image data which is brought to an image pickup by a camera.

CONSTITUTION: On a table 12 on a base 11 of a microscope 10, an object 13 to be brought to an image pickup is placed, and its inspection surface 13a is magnified by a microscope 10 through an objective lens 14 by utilizing a light beam of an illumination 20, and also, brought to an image pickup by a TV camera 15. From its image data, a luminance histogram for showing the number of picture elements to the luminance is derived, and in a prescribed luminance range, luminance distribution of the luminance histogram is divided into two, based on a luminance level as a reference, and with regard to each luminance distribution, a normal distribution is derived. Subsequently, an area (Si) where two normal distributions are superposed, and the minimum value (Sx) of its area are derived, and from a variation in case when a distance of an optical system lens to the object is varied, a point where the minimum value Sx goes to minimum is set as a focused position and the distance is set. Thereafter, a feedback signal is applied to a driving device 19, and a focus of the microscope 10 is aligned automatically by moving a stage 16.