PURPOSE: To utilize the limited capacity of a battery effectively and sufficiently by setting intervals among a plurality of detecting voltage levels so as to be longer on the high voltage sides than on the low voltage sides.

CONSTITUTION: The reference voltage V1∼V5 of a voltage detecting section 3 are set so that the intervals of the high voltage sides are made longer than those of the low voltage sides. The voltage of a battery B for a power supply section 2 is detected by the voltage detecting section 3. A comparison section 4 receives an output from the voltage detecting section 3 and compares the output from the voltage detecting section 3 with an output from an oscillator A, and a base control circuit 5 ON-OFF controls a transistor Q1 by an output from the comparison circuit 4. Accordingly, the power of load L such as an incandescent lamp is controlled to a voltage drop due to the discharge of the battery B.