PURPOSE: To inspect only one surface of a base material and accurately obtain a coating distribution state by irradiating one surface of the coating layer of the base material with light and photodetecting reflected light from the coating layer, detecting selectively light with wavelength absorbed by water as to the reflected light and converting it into an electric signal, and comparing this signal with a reference value.

CONSTITUTION: A windshield W moving on a windshield preprocessing line 7 is coated with a primer containing black ceramic at the peripheral edge part by using a coating robot 8 to obtain a primer layer A and an inspection robot 9 inspects the coating state at the same time. Then the windshield W is moved to an adhesive coating position, a robot 10 is used to coat the primer layer A with an adhesive and forming an adhesive layer B, and the arm 12 of an inspection robot 11 is lowered to position a roller 23 at the tip by the coating layer C. Infrared rays 35, on the other hand, irradiates the coating layer C through a photoconductive cable 30 and are absorbed in proportion to the thickness of the layer B and then reflected to strike on an infrared-ray rotary filter 36, and the generated signal is compared by a comparing circuit 43.