A thermal element of a simple structure which can have a reduced cost because a minimum required number of structural components are used, excels in response and endurance and enables forward and backward movement of a piston with the prescribed stroke under the effect of volume changes following the expansion and contraction of a thermally expandable body. A thermal element 10 containing a thermally expandable body (wax 12) inside a case 11 comprises a piston 13 arranged inside the case along the axial line thereof. The inward end of the piston borders on the inside of the thermally expandable body and the outward end thereof protrudes outwardly from one end of the case, thereby enabling the piston to move forward and backward following the expansion and contraction of the thermally expandable body. The piston is slidably held with a guide member 14 disposed in a portion at one end inside the case. Furthermore, a sealing member 15 is disposed in a portion at the inward end of the guide member inside the case and seals the thermally expandable body at the other end inside the case.