PURPOSE: To detect a frost or dew condensation by a small-sized device, by providding an optical means having the retroreflectivity on the surface to be detected and providing a light projecting element and a light receiving element closed to each other.

CONSTITUTION: A retroreflecting film 7 is formed on the surface of a cooler 1. The film is constituted of fine small globe lens 9 which are mixed in a transparent film 8 and reflecting films 10 are formed on the almost hemispherical parts of the lens 9. When a light 5 is projected from a light projecting element 3 onto the surface of the cooler 1, the light is reflected almost in the same direction by the film 7 and reaches a light receiving element 4. The state of a frost or dew condensation is detected by the change of this light receiving amount. As the element 3 and the element 4 may be provided very closely, the whole device is much compacted.