Abstract: Disclosed is a pen-shaped optical mouse. The optical mouse has an optical fiber on the optical path of illuminating light, and an optical fiber bundle on the optical path of reflected light, respectively. The optical system of the optical mouse is realized irrespective of the optical paths and their lengths, showing a good image-transmitting feature to transmit good image to an image sensor. An image is input to the image sensor through an end of an optical tip where the image directly contacts a reflecting surface, which enhances the accuracy of the pen-shaped optical mouse since there is no change in the location of a mouse pointer or a focal length in spite of the change in the angle between the pen-shaped optical mouse and the reflecting surface.