PURPOSE: To make possible the measuring of the temperature of the surface of an object depending on the position of a reflection image by projecting reflected light beam of a laser light beam hitting a metal object ground thoroughly onto a screen.

CONSTITUTION: A closed metal curved surface body 1 is irradiated with a laser light from a laser oscillator 3 and a screen 2 is arranged at a position where a reflection image from the curved surface body 1 is formed. The curved surface body 1 is provided with a temperature sensor 4, a power source 7 and a thermostat 8. As the temperature of the curved surface body 1 rises, the curved surface body 1 is expanded to change the shape. A light spot of the laser light hitting the curved surface body 1 moves as the temperature of the curved surface body 1 rises. The temperature of the surface of the curved surface body 1 is measured beforehand with the sensor 4 and the surface temperature of the curved surface body 1 is made to correlate with the position of the light spot. The light spot follows one point of a characteristic pattern of a speckle 5 expanding over the screen 2 thereby allowing the learning of the temperature without the use of a thermometer depending on the position of the light spot.