PURPOSE: To automatically detect the surface condition of a transparent substance by measuring the quantity of the light reflected by the transparent substance.

CONSTITUTION: The light-emitting unit 20 and the light-receiving unit 30 are placed on the path through which the light applied to a transparent substance 11 is approximately totally reflected. Thereby, the light emitted by a light-emitting means 12 is radiated as parallel rays by a convergent lens 14, made incident on the transparent substance 11 so as to be totally reflected by its surface, passed through a condensing lens 15 and introduced into a light-receiving means 13. When the surface of the transparent substance 11 is cloudy, the minute water drops on the surface cause the light to scatter on the reflecting surface of the transparent substance 11, so that the quantity of the light entering the light-receiving means 13 decreases. Accordingly, measuring the quantity of the light entering the light-receiving means 13 permits the surface condition of the transparent substance 11, such as cloudiness, to be automatically detected.