A linear heater is formed of an enamel wire composed of a heating wire and an enamel layer. The heating wire is made of a copper alloy containing silver, for example. The enamel layer is made of polyester imide (PEI), polyimide (PI) or polyamide imide (PAI), for example. The enamel layer is coated with an insulating coating layer. The insulating coating layer is made of fluororesin such as perfluoroalkoxy mixture (PFA), polyimide (PI), or polyamide imide (PAI). The linear heater is bonded to the lower surface of an upper toilet seat casing such that it is sandwiched between a metal foil and a metal foil made of aluminum, for example.