PURPOSE: To automatically turn off an electrical source when the balance is out of use for longer than a predetermined time period to reduce power consumption by using a constitution in which the power source is automatically turned off when the load detection data remains near zero for longer than a predetermined period and again turned on upon actuation of a tare deduction command key.

CONSTITUTION: When a near-zero value is supplied after turning on of a power switch 7 to a control part 3 from a load sensor 1 through an AD converter 2, the control part 3 judges that the balance is in a non-use state and transmits a judgement result signal J to an FF 12b of a power source on/off control circuit 12. The FF sends a low level control signal C to a contactless AC relay 11 which is automatically turned off so that the power supply is terminates by a main current circuit 9. On the other hand, upon actuation of the tare command key 5, reset signals are sent to the FF 12b from the control circuit 3 so that the signal C goes to H level to turn on the relay 11 to again initiate power supply from the circuit 9.