An arc-welding quality judging system includes a setting part which sets an upper limit value at a first time based on the maximum value at the first time or at a plurality of times containing the first time at every first time, and sets a lower limit value at the first time based on the minimum value at the first time or at the plurality of times containing the first time, in a prescribed population. The arc-welding quality judging system further includes a judgment part which, when a measurement value obtained upon welding exists between the upper limit value and the lower limit value, judges that there is no defective welding and, when the measurement value exists outside the range, judges that there is a defective welding.