A focusing device for an optical microscope may include a light emitting unit configured to emit laser light having a specific wavelength, a wedge mirror configured to enable the emitted laser light to be incident on a plurality of locations of a surface of a specimen, first and second light receiving units configured to detect an amount of laser light reflected from the surface of the specimen, a spatial filter configured to eliminate out-of-focus light from light beams reflected from the surface of the specimen and to detect an amount of in-focus light, and a control unit configured to generate a control signal used to carry out focus adjustment of the optical microscope using a plurality of light-amount information detected by the first and second light receiving units and the spatial filter.