PURPOSE: To make it possible to measure the temp. of a remote place having no external power source in a high S/N ratio, by utilizing the temp. dependency of the response speed of a liquid crystal.

CONSTITUTION: Lights of a light emitter 1 are divided into two parts while sent to a light distributor 4 through a light sending optical fiber 3 and one of them is made to irradiates a photo-electric cell 5 to drive a liquid crystal 6 by the output of said cell 5 while the other of them is made to irradiates the single surface of the liquid crystal 6. In this case, a response time T1 reliable to a temp. is resent in the liquid crystal 6 which is, in turn, brought to a light pervious state after the time T1 is elapsed from the application of voltage and the pervious light is applied to a light receiving diode 9 through a light receiving optical fiber 7. The output of EX-OR12 receiving the output from said diode 9 is applied to a mono-multiplier 13 and the light emitter 1 is controlled by the output therefrom. This closed loop forms an oscillation circuit and the oscillation frequency thereof is counted by a frequency counter circuit 15. By this method, the temp. of a remote place having no external power source can be measured in a high S/N ratio.