PURPOSE: To surely inspect the adequateness or inadequateness of the coating width of a high-viscosity coating material without erroneous recognition.

CONSTITUTION: A light source 2 and a photodetecting section 3 are arranged apart from each other and a light shielding member 4 is disposed therebetween. The light source 2, the photodetecting section 3 and the light shielding member 4 are disposed in such positional relations that these parts are included in a light shielding area (shown by hatching) formed with at least the peak part 1a of the surface of the coating material 1 behind the light shielding member 4 and the boundary apart 5a from at least the coating material 1 of the coating surface 5 is included in the area to be projected with the light from the light source 2.