To realize a high-speed and high-stability autofocusing which is effective to a microscopic system such as a laser scanning microscope.

In the case of controlling autofocusing by condensing a laser beam 19 by an objective lens 12, irradiating a substance to be measured 13 with the laser beam, detecting reflected light beam and moving the substance 13, the reflected light beam is detected by successively moving the substance 13 to three measuring points separate at equal intervals by using a moving stage 17 so as to discriminate the maximum value of the intensity of the reflected light beam and the position thereof. Next, the reflected light beam is detected by successively moving the substance 13 to three measuring points, that is, the discriminated position and two positions at a distance being about half of the previous interval centering that position. By repeating the observation and the comparison of the intensity of the reflected light beam at three measuring points while decreasing the interval between the measuring points and continuing them until the interval between the measuring points becomes the same extent as the resolution of the moving stage 17, the best focusing position where the intensity of the reflected light beam is the maximum is attained.