Abstract: A dust preventing device for an electromagnetic contactor prevents foreign matters accumulated on a foreign matter preventing wall from slipping off the wall due to vibrations of electromagnetic contactor. In a case where an arc extinguishing chamber cover is disposed to cover a fixed contact and a movable contact, and includes a foreign matter preventing wall provided under a gas discharging opening formed on the cover, the foreign matter preventing wall is formed with ribs provided therearound in a shape of an embankment. Therefore, foreign matters having entered through the gas discharging opening can be positively trapped in a hollow portion surrounded by ribs, so that the foreign matters do not fall into the electromagnetic contactor from the foreign matter preventing wall due to vibrations of the contactor.