To detect a in-focal-point even though contrast values of a front focusing image and a rear focusing image are uneven by providing a means judging an in-focus by the inquiry whether the distance between the relative position between a moved stage and an objective lens and a stored relative position coincides with the distance calculated by a distance calculating means or not, etc.

When the contrast evaluation value of a front focusing image is larger (smaller) than that of a rear focusing image, a stage 1 is moved along an observation axis by a prescribed amount by a driving circuit 16. A position of the stage 1 where the contrast evaluation value becomes maximum is searched by repeating the moving of the stage until the contrast evaluation value of the front focusing image is lowered as compared with that before the moving of the stage 1. This position of the stage 1 is stored in a memory 17 together with the maximum value of the contrast evaluation value of the corresponding front focusing image and the distance between positions of the stage at the point of time when contrast evaluation values of the front and rear focusing images coincide is calculated by a subtraction. The coincidence between this distance and a theoretical value is judged and the position of the stage 1 at the point of time when they coincide is judged to be an in-focal-point.