Abstract: A thermo-element having an excellent response and durability is provided. The thermo-element includes a casing having a bottom, a thermally expandable material filled in the casing and capable of expanding and contracting due to temperature changes, a piston movable in an axial direction thereof, a guide member for slidably holding the piston, and a liquid chamber defined between the thermally expandable material and the piston to receive therein a deformable, incompressible fluid. A bellows is fixed between an upper portion of the piston and an upper portion of the guide member to define a liquid space between an outer periphery of the upper portion of the piston and the bellows. The fluid in the liquid chamber can flow in and out of the liquid space via a through-hole and a radial hole.