PURPOSE: To prevent disordered operation of a microcomputer, by disabling a circuit element to be controlled when the applied voltage of the circuit element becomes lower than a specified voltage level, and at the same time, resetting the microprocessor.

CONSTITUTION: An output of a voltage comparator circuit 3 becomes an L level when the voltage of a constant voltage power source 5 becomes lower than the input voltage (specified value Vcc) to be applied to a microprocessor (MPU) 6, and a write and read memory (RAM) 7. At this time, the RAM7 becomes a disabled state (impossible to write and read). At the same time, the charge of a capacitor connected to the reset terminal of the MPU6 is discharged through a resistance 15 and a transistor 14. When the time of voltage drop is long, the MPU is reset surely. Even when the voltage drop is momentary and imperfect, the MPU is reset surely and quickly by operation of a discharge circuit. Accordingly, disordered operation of a welding machine due to malfunction of the MPU and erroneous reading of the RAM can be prevented perfectly.