Abstract: According to the present invention, there is provided a thermoelement that suppresses the corrosion or corrosion products occurring on an outer peripheral surface of a piston and an inner wall of a guide portion so that the piston reciprocates surely and smoothly, and a thermostat apparatus using this thermoelement. The thermoelement according to the present invention includes a wax case that internally has a wax, which is an expanding body, a piston to which responsive action of the expansion and contraction of the wax is transmitted, and a guide portion that guides the sliding of the piston, wherein the guide portion is made of brass containing 69 mass % or more and 80 mass % or less of copper component, 2 mass % or more and 4 mass % or less of silicon component, 0.1 mass % or less of lead component, and the balance of zinc, and the piston is made of stainless steel.