To enable learning of a timing of replacing a welding electrode before a defective product is generated.

An allowable limit range of quality of a welding electrode is defined as a range S1 prescribed by a welding current acceptance upper limit I_OKU and a welding current acceptance lower limit I_OKL, and a control limit range of the quality of the welding electrode is defined as a range S2 prescribed by a control upper limit I_CSU and a control lower limit I_CSL. The control limit range S2 is set within a range of the allowable limit range S1. A maximum value Ip of the welding current I is obtained on every welding. When the obtained maximum value Ip of the welding current I is in the allowable limit range S1 and is out of the control limit range S2 for N times (for example, three times) in a row, it is determined that replacement of the welding electrode is necessary.