PURPOSE: To prevent the collision of a sample with an objective lens.

CONSTITUTION: In a confocal scanning optical microscope comprising an objective lens 7 converging a transmitted light beam from a light source 2 on a sample 8, a photodetector 13 detecting a beam from the sample 8, a scanning mechanism 5 two-dimensionally and relatively scanning the light beam converged on the sample 8 and the sample 8 and a moving mechanism for relatively scanning the focal position of the objective lens 7 and the position of the sample 8 in the direction of the optical axis, in order to decide the observing range or the measuring range of the sample 8, first, only the movement in the direction in which the objective lens 7 approaches the sample 8 is permitted, when the focal position of the objective lens 7 is moved to the uppermost surface of the sample 8, this position is made to be an upper limit, the movement in the direction in which the focal position of the objective lens 7 is approached to or separated from the sample 8 is succesively permitted so that the position on which the focal position reaches the lowermost surface of the sample 8 is found and the function for stopping the movement is given when the moving amount exceeds the working distance of the objective lens 7.