To obtain a light level sensor having simple structure and excellent in sensitivity and durability.

A light guide object 4 for coupling a light emitting element 2 and a light receiving element 3 optically is provided, in the way thereof, with a sensing part 5 filled with a substance for scattering propagating light end radiating a part of the scattering light from the interface over a range of length L selected while taking account of the liquid level difference being predicted. Consequently, when the light emitted from the light emitting element 2 and propagated through the light guide object 4 is detected by the light receiving element 3, liquid level corresponding to attenuation of the propagating light at the sensing part 5 can be detected.