In a method for detecting a deviation in arc-tracking welding, a deviation between a welding line and an actual welding position is detected in arc-tracking welding for performing welding tracking the welding line while performing a weaving operation of swinging a welding torch with respect to a welding direction. The method includes fitting a waveform expressed by a function repeated periodically in a period equal to a weaving period to a waveform of a welding current or a welding voltage and detecting a deviation in arc-tracking welding based on a fitted waveform.