Avoiding an illumination light irregularity to the utmost under increasing degree of integration and the memory capacity by suppressing a light amount irregularity on the illumination pupil plane to the utmost, unable to be adjusted by a conventional method. When the light amount irregularity on the illumination pupil plane which is the exit surface of the optical fiber is relatively large, focus of the imaging surface of the CCD camera is switched from the sample to the illumination pupil plane by a focus switching lens. The light amount irregularity is calculated and analyzed by the image processor. Based on the analyzed result, the exit surface position of the optical fiber is adjusted in the illumination optical system. In this manner, the illumination pupil plane which is the exit surface of the optical fiber can be adjusted to an illumination pupil plane where the light amount irregularity is small.