A seat heater includes a seat-warming heater which is connected to a power source and warms a person sitting on a seat, an over-temperature-preventing part which disconnects the connection between the seat-warming heater and the power source when the over-temperature-preventing part is heated to a predetermined temperature so as to prevent over-temperature of the seat-warming heater, and a heater which heats the over-temperature-preventing part, wherein the over-temperature-preventing part is non-return type, and the heater for the over-temperature-preventing part is arranged to cover opposite portions of the over-temperature-preventing part, thereby realizing both higher output and improved safety.