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A practical fiber-optic measurement instrument for temperature was constructed consisting of a small sensor responding to optical absorption change in a semiconductor, and a unique signal processing system with two different-wavelength light emitting diodes (LED's). The fiber-optic sensor with a semiconductor chip is quite small, very sensitive, highly reliable, and easy to manufacture at low cost. The most outstanding feature of this system is that it is free from optical-stray-loss. The accuracy of about +-1° and the response time of about 2 s were obtained in the temperature range from -10° C to 300°C.