A light emitting element and a light receiving element is placed in such a manner that the light axis of a light emitting element constructing a light emitting circuit and the light axis of a light receiving element constructing a light receiving circuit cross each other. The light pathway length R1, R2 of both the light waves are determined in such a manner that the object light, the light waves via the object pathway, are delayed by π/2 in phase from the reflected light, i.e. relative to light waves via the reflected light pathway. A synchronous detector selects signals having the same phase as the object light from receive signals provided from the light receiving circuit. Accordingly, the fog detector can detect the object light without being influenced of the reflected light, which allows a determination processing device to accurately determine the density of fog.