To optimize focusing time and focusing accuracy in accordance with various microscopic methods.

The microscopic method is switched to either a bright field microscopic method or a fluorescent microscopic method for a subject S, the optical image of the subject S is formed on a CCD sensor 17 through an objective lens 3, and an electrical signal outputted from the sensor 17 is amplified through an AGC 20, then focusing operation is performed by varying a relative distance between the subject S and the lens 3 based on the electrical signal. In such a case, a CPU 24 is provided with a function for releasing the restriction of the storage time of the sensor 17 and restricting the control range of the gain of the AGC 20 in the case of discriminating the bright field microscopic method, for example, as the microscopic method discriminated by an optical path detector 25, and restricting the charge storage time of the sensor 17 and releasing the restriction of the gain of the AGC 20 in the case of discriminating the fluorescent microscopic method.