PURPOSE: To maintain the welding quality satisfactory by monitoring a resistance value at the time of welding by comparing pulse width of a detection welding current and a welding current value at an arbitrary time point set in advance in the pulse width with a reference value.
CONSTITUTION: After the lapse of prescribed time after welding is started, an output of a current detector 28 is subjected to digital conversion by an A/D converting circuit 44t and stored in a second storage means 54 under the control of a control means 66. Based on information of a stored welding current value, a numerical processing is executed by a variable arithmetic means 56 and a pulse width arithmetic means 58, and it is stored in a third storage means 60. Pulse width of a stored welding current with a reference value stored in a first storage means 50 are compared by a comparing means 62. When a result of decision is within an allowable value, sampling of the next welding current is started. When the result of decision is out of the allowable value, a buzzer of an alarm means 64 is sounded.