The linear grating coupler of the optical waveguide element is sensitive to the incident beam angle. This characteristic was considered a weakness when used in a conventional optical measurement system. However, for the inclination measurement, this becomes a valuable characteristic. For example, to obtain higher optical performance in semiconductor lithography system, inclination control of the wafer substrate surface is becoming more important. And a wafer inclination measurement optics attached to the stepper requires high measurement accuracy, ability to deal with patterned and photoresist coated wafer, and compact size and a linear grating coupler has a possibility to satisfy these conditions. To verify inclination measurement ability of the linear grating coupler formed on the optical waveguide element, an experimental system was produced. A collimated spolarized He-Ne laser illuminated the wafer surfaces and coupled beam intensity of the reflected or scattered light from the tested surface was detected. Inclination of the substrate was obtained from the maximum intensity position. Standard deviation of the measurement accuracy of the order of 10-6rad was confirmed even when a patterned and photoresist coated wafer was measured. The experimental results fit the numerically simulated results.