To provide a thermoelement that prevents a sealing member for sealing wax from deteriorating and has high responsiveness and durability.

Grooves (41) are formed vertically on an inner surface of a piston insertion bore of a sleeve (40) of the thermoelement. Alternatively, vertical ribs (42) are formed on an outer surface of the sleeve. When the sleeve is crushed, the sleeve is bent at the grooves or ribs and is not bent irregularly. The piston insertion bore of the sleeve, in a shape where the sleeve is formed, has an inner diameter (40d) equal to or larger than an outer diameter (6d) of a piston (6), or the piston insertion bore has an inner diameter smaller than the outer diameter of the piston. In the latter case, the crushing of the sleeve can be reduced, when the piston moves up.