PURPOSE: To detect the temperature of a substrate without contact, by projecting light on a bimetal provided on the surface of the substrate, and detecting the displacement of the reflected light based on the bending of the bimetal due to the change in temperature.

CONSTITUTION: A bimetal 45 comprising two kinds of raw metal materials 4 and 5 is formed on a substrate 1. Light emitted from a light projecting device 6 becomes incident light 71 through a light-path changing device 9. The light 71 is reflected by the surface of the substrate 1 and becomes reflected light 72, which is inputted to a position sensor 8. The output is inputted to an amplifier 10 as an output V1 as a function of the position of the incident light. Then the light-path changing device 9 is rotated by a specified angle, and incident light 75 is reflected by the bent surface of the bimetal 45. An output V2 is obtained from reflected light 73. When the temperature of the substrate 1 is changed, the inclination of the bending of the bimetal 45 is changed, the reflected light 73 becomes 74, and an output V3 is obtained. The temperature of the substrate 1 is detected without contact by the difference between V1 and the outputs V2, V3.