The present invention provides a switch mechanism usable underwater including a rear cover serving as an exterior member, two shaft members being displaceable in a shaft direction and fitting into two shaft holes having the same cross-sectional area provided in the rear cover, an O-ring watertightly maintaining the shaft member and the shaft hole, and a lever member being engaged with the two shaft members in a manner to be capable of pressing-down or pulling-up the shaft member, and being configured to balance water pressure applied to one shaft member with that applied to the other shaft member.