Terminal bolts are inserted in the bottom of contact cover through collars. C-rings are fitted into U-shaped grooves formed in the terminal bolts and constitute locking parts. Circular notch parts are formed on the innersurface at the contact chamber side of the collars and the notch parts are engaged with the C-ring whereby ring terminals receive pressing force of nuts tightened on the terminal bolts. Accordingly, spaces are created at places being closer to the contact chamber side than the C ring side of the terminal bolts are, by interposing O-rings with large thickness in the spaces in elastically compressed states, whereby an airtightness of the contact chamber can be secured without expanding an outer diameter of the contact cover. Thus, an electromagnetic switch can adopt sealing members with sufficient thickness without expanding an outer diameter of the contact cover.