PURPOSE: To simplify a circuit through selection of a desired calorific value and the use of a heat generating also serving as a signal wire by a method wherein the one of first and second heat generating wires having different calorific values is switched to a power source for connection and the other is switched to an overincreasing temperature preventing means for connection.

CONSTITUTION: In the initial stage of energization, a first heat generating body H1 is connected to a power source 10 and a second heat generating body H2 is connected to an overincreasing temperature preventing circuit formed with a heat generating circuit RH, a diode D2, a circuit protecting resistor R1, and a fuse 12. When, under a state in which the temperature of a carpet body is increased, the temperature exceeds e set temperature, relay contacts RA, RB, RC, and RD are switched by a temperature control circuit 19, the second heat generating body H2 is connected to the power source 10 and the first heat generating body H1 to the overincreasing temperature preventing circuit. Thus, when, during the initial stage of energization, the temperature of the carpet is rapidly increased by the heat generating body H1 having a high calorific value, the heat generating bodies H1 and H2 may serve also as a signal wire, and constitution of a circuit can be simplified.