In an optical unit used for a laser beam printer, a laser beam generated by a laser diode is converted by a group of conversion lenses into a laser beam having a predetermined-size cross section. The laser beam is directed toward a photosensitive body by a scanner. A focusing lens allows the rotating angle of the reflecting faces of the scanner to correspond to a desirable point on the surface of the photosensitive body. In other words, the rotating angle is made to correspond to the distance between the optical axis center determined with respect to a main scanning direction and a point to which the laser beam is irradiated for scanning. The conversion lenses includes at least one aspheric glass lens arranged close to the laser diode. The glass lens and the laser diode are integrally held by a lens barrel which linearly expands in a predetermined manner in response to a temperature change. Since the distance between the glass lens and the laser diode is constantly equal in accordance with the temperature change, the laser beam is constantly allowed to have a cross section of desirable size, without reference to the temperature change.