To provide an appropriate nugget diameter by judging that the welding is inappropriate when the actual nugget forming starting time is longer than the nugget forming starting time of the target estimated nugget diameter in the delay standard.

The corresponding table is read from a nugget diameter forming starting time storage device, and an appropriate nugget forming starting time is set as the comparison reference based on the target estimated nugget diameter. The voltage, the current, etc., are measured by a detection part 10 when electrodes 1, 2 are energized, and the energization time is compared with the nugget forming starting time. When the energization time is within the nugget forming starting time of the target estimated nugget diameter, a monitor 8 displays that the welding is appropriate, the monitor 8 displays that the welding is inappropriate when long time. Acceptance/rejection of the welding can be judged.