PURPOSE: To facilitate the setting of independent filters with respect to visible light and infrared rays, by forming an image on the surface of the same compound detector passing through a wide-band filter for the infrared rays and through a narrow-band filter for the visible light.

CONSTITUTION: A visible light V incident into a primary mirror 9 is reflected thereon and then on a reflecting surface 11a of a lens 11 through a lens 10 to form an image on a compound detector 8 passing through a filter 13. Infrared rays R is focused with a lens 12 to form an image on the compound detector 8 passing through the lens 11 and the filter 13. After passing through independent optical paths, visible light V and infrared rays R form an image on the common compound detector 8. Thus, the reflecting surface 11a for reflecting the visible light V is made planar to facilitate the manufacture of the narrow- band filter for infrared rays R by the existing multilayer evaporation technique.