To investigate the corrosion mechanism of magnesium alloys under atmospheric conditions, field exposure tests were performed. In a marine environment, AZ31B and AZ91D alloys experienced localized corrosion, including pitting and filiform corrosion. In order to reproduce such actual atmospheric corrosion in a laboratory experiment, we employed a wet-dry cyclic test at a constant dew point. From the results of the wet-dry cyclic tests, the propagation mechanism of localized corrosion on magnesium alloys has been discussed. Localized corrosion occurred during the wetting period, and the corroded sites thus formed were repassivated during the drying period. In the next wetting period, new localized corrosion took place, but the repassivated sites were not reactivated. The repassivated sites have higher corrosion resistance than the non-corroded part of the surface because of the barrier effect of the corrosion products. In the case of AZ91D, Al enrichment in the surface region of the metal matrix is responsible for this corrosion resistance.