To enable more exact focusing of an optical microscope, regardless of surface roughness of a sample, and always obtain optimum focusing accuracy.

This device is provided with a sample stage on which a sample is mounted, a stage control part for driving the sample stage, an objective lens for an optical microscope fixed and arranged relative to the surface of the sample in the optical axis direction, a projection pattern member 21 projecting patterns on the surface of the sample via the objective lens, a sensor detecting images of the projection patterns projected on the surface of the sample formed via the objective lens and an automatic focusing controller for controlling the stage control part so as to automatically focus a focus of the optical microscope based on a detection signal of the sensor. In this case, the projection pattern member 21 is constituted of narrow interval pattern parts 21A and wide interval pattern parts 21B.