The welding quality of materials sandwiched between and welded by two electrodes is monitored by a welder monitor, which includes a detecting section for detecting a welding current and a welding voltage, an operation section for numerically analyzing, based on a heat conduction model, a temperature distribution of the two electrodes and that of the materials using the detected welding current and welding voltage, to thereby estimate a nugget diameter during welding, a reference diameter setting section for setting a reference nugget diameter, and a diameter comparing section for comparing the nugget diameter during welding with the reference nugget diameter, to thereby compute a time at which the nugget diameter exceeds the reference nugget diameter. The welder monitor also includes a reference time setting section for setting a reference time shorter than a given time until which welding of intended materials is not terminated, a time comparing section for comparing a nugget formation time, at which the nugget diameter during welding exceeds the reference nugget diameter, with the reference time set by the reference time setting section, and an output section for outputting a comparison result from the time comparing section. When the nugget formation time is shorter than the reference time, the nugget formation time is outputted to the output section.