To provide a clinical thermometer with an opening cover for closing an opening at the tip of the probe part to be manufactured easily with little wasteful material and hardly broken, and with reduced error of measurement by preventing transmission of heat to a light guide tube.

A cushioning/heat insulating ring 15 is fixed to the rim of an opening cover 18 of the clinical thermometer. A cushioning/heat insulating ring presser part 16a is formed at the tip of an opening cover fixing body 16, and the cushioning/heat insulating ring 15 is clasped between the cushioning/ heat insulating ring presser part 16a and the tip of a light guide tube 13. By engaging a female screw cut at the base of the opening cover fixing body 16 with a male screw cut around the light guide tube 13 under the above condition, the opening cover 18 is fixed to the top of the light guide tube 13. A gap 20 is formed between the outer surrounding surface of the light guide tube 13 and the inner surrounding surface of the opening cover fixing body 16. The opening cover 18 is formed in the shape of an equilateral hexagon.