PURPOSE: To simplify a controlling circuit, and to decrease the number of parts to be used, by digitizing a control function required for an arc welding machine.

CONSTITUTION: A digital IC 31 contains functions of a sequence circuit, a pre- flow timer, an after-flow timer, a waveform shaping circuit and a displaying circuit. The first A/D converter 32 converts a set value of a pulse frequency setting device 12 to a digital value, the second A/D converter 33 converts a set value of a pulse width setting device 11 to a digital value, and each of them is inputted to the IC 31. Also, a digital pre-flow time setting device 34 and a digital after-flow time setting device 35 input each set value to the digital IC 31. The third and the fourth A/D converters 38, 39 convert a welding current and voltage to digital values, respectively.