To monitor presence/absence of an abnormality by providing at least a setting function of the current value and the energization time at each stage, the indicating function of the value by this setting, and the indicating function of the measured value of the current on a setting unit of the energization condition to indicate the measured value of the current at each stage if the current value and the energization time are set.

When the measured value of the current is deviated from the allowable range of the set value of indication frames 12, 13, 14, and the excessive current flows in a local welding part, the spatter is scattered, and the welding quality is degraded. When such a condition is found, the current value is corrected to prevent generation of the spatter. When the current value in the local welding part is insufficient, a screen concerned is called, and the current is increased so as to be close to the set value. When the current is greatly dropped, an abnormality can be present, such as defective wear of an electrode tip and stain of a steel plate parts. Since the stepwise control includes at least three steps, i.e., the preheating energization, the melting energization and the regulating energization, and is effective for the countermeasures for the spatter generation.