PURPOSE: To adjust automatically the fluctuation in a light receiving level due to temperature change of an optical fiber by winding an optical fiber on a thermal expanding cylinder.

CONSTITUTION: A rugged sand paper 11 is wound on an outer circumferential face of the thermal expanding cylinder 10. Further, an optical fiber 12 is wound on the sand paper 11. Moreover, a connector 13 is connected to both ends of the optical fiber 12. Through the constitution above, since an external force is weakened to the optical fiber 12 wound on the cylinder 10 via the sand paper 11 at a prescribed tension as the temperature is lowered and the loss by micro bent is reduced and the transmitted quantity of light is increased, the change in the light receiveing level attended with the temperature change is adjusted automatically.