PURPOSE: To generate a sensitive voltage only in part of a temperature range by providing a switching circuit to connect a first transistor(TR), which is turned on at a temperature T1, and second TR, which is turned off at a temperature T2, and to operate in the temperature range of T1 to T2.

CONSTITUTION: A thermister TR1 and a resistance R4 are set so as to be turned on at the temperature T1. A resistance R5 and a thermister RT2 are set so as to be turned off at the temperature T2. By turning on and off first and second TRs TR1 and TR2, switching circuits TR1 and TR2 are operated in the range from the temperature T1 to T2. Consequently in the partial area between the temperature T1 to T2, a sensitive voltage V2 is set so as to conform to the area, and in the area other than the partial area, a normal voltage V1 at a higher level than that of the sensitive voltage V2 is set. The values of the normal voltage V1 and the sensitive voltage V2 can freely be set by resistances R1, R2 and R3.