The size and production costs of an optical microscope apparatus capable of blocking light or maintaining the specimen environment are reduced. The provided optical microscope apparatus includes a microscope that has a stage for mounting a specimen (A), a transmission-illumination optical system, and a detection optical system; and a housing that surrounds the microscope, wherein the housing includes a fixed housing, and a movable housing, wherein, among optical parts constituting the transmission-illumination optical system and the image-forming optical system, at least some optical parts disposed above the stage are movable, and wherein a switching mechanism is provided, the switching mechanism being configured to retract the optical parts away from above the stage when the movable housing is disposed in an open position relative to the fixed housing and to substantially align the optical axes of both optical systems when the movable housing is disposed in a closed position relative to the fixed housing.