An optical reflecting unit comprises a support prism extending in an elongated manner, and a half mirror fixedly mounted in contact with a surface of the support prism. An optical axis of the incident light in the form of a line incident upon a half mirror is folded. A transmitted light transmitted through the half mirror is incident upon the support prism. An extracted bore is provided in which an incident transmitted light is irregularly reflected by a bottom and a peripheral wall of the extracted bore. With the arrangement discussed above, the transmitted light transmitted through the half mirror and reflected by the surface of the half-mirror support prism is not deviated in a direction along the optical axis of the reflected light from the half mirror. Only the reflected light from the half mirror can be led to a predetermined direction.