A light of measurement emitted from a semiconductor laser (1) is incident to a beam splitter formed by a prism (4). The light of measurement transmits into an incident surface (3) having a positive power so as to be emitted to a reflecting surface (5c) of measurement. A light reflected there is returned by a beam splitting surface (4a) so as to be reflected by a concave reflecting surface (6). The reflected light is returned so as to be condensed and focused on a light receiving surface (8) of sensor in a light measuring device (7). A position of a spot of light which is received is measured so as to measure the deflection angle of a rotating mirror (5). By doing this, it is possible to form a compact device even if the length of the optical path is long. As a result, it is possible to broaden the deflection angle measuring range.