In a switch device, a switch is provided in a tightly closed switch chamber in a casing, and an operating element is advanceably and retreatably inserted into the switch chamber through an opening in the casing. An actuating member, connected to an inner end of the operating element for operating the switch, is provided in the switch chamber and has a fitting hole which including a small diameter portion and a large diameter portion. The inner end of the operating element is passed through a sealing member for sealing between the operating element and the opening, and is fitted into the small diameter portion of the fitting hole to form an annular seal clamping portion between an inner peripheral surface of the large diameter portion of the fitting hole and an outer peripheral surface of the operating element. An outer peripheral surface of the sealing member is secured to the casing, and an inner peripheral surface of the sealing member is clamped by the seal clamping portion. Thus, it is possible to insure an intended sealing function for a long period of time.