To lower the production cost by reducing the number of part items in a thermo-element, to shorten a size in the longitudinal direction of a piston for miniaturization, to prevent a sliding part of this piston from being exposed to the outside and also a thermosensitive member from leaking out of this sliding part, to check any penetration of water into the sliding part, having no adverse effect on it, and further to make any adverse effect so as not to be given to a slide motion without stacking water scales to the sliding part.

This thermo-element is so constituted that one end 3a of a piston 3 is supported free of sliding motion on a piston guide 2 installed in a case 1 filled up with a thermosensitive member 4 being expanded or contracted by temperature change, and a deformative elastic member 5 transmitting any expansion of the thermosensitive member 4 to the piston 3 is locked of its outer circumferential part 5a to an opening part 1a of the case 1, while its inner circumferential part 5b is held between a tip part 3b of the piston 3 and an outer piston 6 and simultaneously the thermosensitive member 4 is made so as to be hermetrically sealed in the case 1.