PURPOSE: To eliminate an electric accident by a complete insulation of the control circuit section of a temperature control detector by moving a movable plate in response to the temperature rise of substance to be detected at its temperature and optically detecting the movement of the plate when the moved distance of the plate reaches predetermined value.

CONSTITUTION: Liquid 2 to be detected at its temperature such as hot water is stored in a tank 1. An opening is perforated at the upper portion of the side wall of the tank 1, and a container 3 making contact with the liquid 2 at its head is inserted into the opening. The coupled portion of a thermocouple 4 is filled sealingly in the container 3 to detect the liquid temperature in the tank 1. The cold coupled point 5 of the thermocouple 4 is connected to two stationary coils 6, 6 connected in series to thus supply the electromotive force of the thermocouple 4 to the coils 6, 6 to energize the coils 6, 6. When the liquid temperature is raised now, the coils 6, 6 are energized to rotate a permanent magnet 7. Thus, a movable plate 9 is rotated. Accordingly, the light from a light emitting element 12 is applied to a photoelectric converter 13, which thereupon produces a signal to a control circuit 14. Thus, this detector can detect that the liquid reaches predetermined temperature and thus stop heating of the liquid 2, for example.