To enhance temperature detection accuracy at low cost in a simple constitution.

Many projecting parts 17 or the like are formed at an inner circumferential face of a cylindrical body 16 or the like of a material having a smaller thermal expansion coefficient than that of an optical fiber 11. A local part of the optical fiber 11 inclined to be thermally expanded is clamped by top parts of the projecting parts 17 to the inside in a diametrical direction, thereby to change an outer circumferential face of the optical fiber 11 to be rough so as not to totally reflect the inside light. The amount of a projecting light from the optical fiber 11 can be changed greatly in accordance with a temperature change even without using a shape memory alloy. Accordingly, temperature detection accuracy is improved at low cost.