A waterproof structure with a simple structure and a good assemble workability is provided. A recess portion is formed on a side of an inner casing, and a through hole is formed at the bottom of the recess portion. The through hole runs to a housing recess portion, and a dome switch opposes to the through hole in the inner casing. A protrusion of a cup member is inserted into the through hole from the outside of the inner casing. A protrusive portion protruding from the bottom surface of a push button is inserted into the opening of the cup member. A rubber sheet which couples the push button is sandwiched between the inner casing and a top cover and a back cover.