a first sensor and a second sensor configured to receive an optical image of a target workpiece to be inspected and photoelectrically convert the optical image of the target workpiece;

a first optical system configured to focus the optical image on the first sensor;

a second optical system configured to branch the optical image from the first optical system, and focus the branched optical image on the second sensor;

a focus detection part configured to input first image signals photoelectrically converted by the first sensor and second image signals photoelectrically converted by the second sensor, to differentiate the first and second image signals respectively, to calculate frequency distribution with respect to differential values of the first image signals and differential values of the second image signals respectively, to calculate density averages of frequencies of N % or more respectively, and to detect a focus position of the optical image by using the density averages of frequencies of N % or more as high frequency components of the first image signals and the second image signals;

a focus control part configured to control a focus of the first optical system based on the focus position detected by the focus detection part; and

a comparison part configured to compare the first image signals photoelectrically converted from the focused optical image and predetermined reference image signals.