A pen like optical mouse includes a pen like mouse main body, a printed circuit board disposed in the main body in a direction parallel to a central axis of the main body, a light emitting element mounted on the first printed circuit board, a sensor die mounted on the second printed circuit board, and an imaging system for guiding light emitted from the light emitting element to be irradiated onto and reflected by a work surface and then guiding the reflected light so as to be imaged and incident on the sensor die. As the light emitting element and the sensor die are arranged on two separate printed circuit boards, light emitted from the light emitting element is prevented from being directly incident on the senor die so as not to reduce the contrast of images.