PURPOSE: To increase the attenuation factor of a light source for incidence and to improve the reliability of a reference light source by forming the reference light source in a V shape.

CONSTITUTION: The reference light source 15 consists of a light shield plate 7 which is arranged on the reverse surface of a reflecting mirror 5 opposite a rotary polygon mirror 2 and a multi-element infrared detector 1, and a light reflecting surface surface 11 facing the light shield plate 7. The light shield plate 7 and reflecting surface 11 consist of a V-shaped formation part 6 where their one-end sides cross each other on their prolongations. Consequently, light 18 which is incident on the V-shaped formation part 6 of the reference light source 15 through the mirror 2 and a reference light source window 8 as shown by an arrow is attenuated gradually while reflected by the light shield plate 7 and light reflecting surface 11 successively and nearly absorbed by the light shield plate 8 and light reflecting surface 11 when reaching the vertex of the V-shaped formation part 6.