A contaminant proof structure for a starter for an engine, with the starter including a sliding portion between a sliding member and a support member supporting the sliding member disposed in a housing so that the sliding member can project into the outside of the housing through an opening formed in a wall of the housing. An atmospheric gas reservoir disposed in the housing between the support member and the wall of the housing around the opening, and a communication passage bypasses the sliding portion and communicates the atmospheric gas reservoir with an external atmospheric gas through a space at an opposite side of the support member to the atmospheric gas reservoir side. The sliding member is dimensioned such that a gap between the opening and the sliding member inserted in the opening is small.
