To achieve an element assembly with a thermoelement of a thermostat device that has a simple structure and excellent assemblability and attains reduction in size and weight while having excellent durability, excellent wear resistance and the like.

The thermoelement 21 is configured such that a thermal expansion body 32 expanding/contracting due to temperature changes is built in a case 31. The thermoelement includes: a piston 33 that is arranged along the axial direction inside the case in which its inner end faces the inside of the thermal expansion body and its outer end protrudes from an opening part on one end side of the case to the outside so as to advance/retreat due to expansion/contraction of the thermal expansion body; a guide member 34 that is arranged while being inserted into the inside of the case from the opening part on one end side of the case so as to freely slidably hold the piston; and a poppet-like first valve element (a poppet-like member) 22 fitted to the outside near the opening part on one end side of the case so as to open/close a fluid passage. A part 41 press-fitted to the outside of the case and the first valve element and parts 42, 43 inserted into the inside of the case of the guide member are arranged by deviating their positions in the axial and radial directions of the case.