An electronic device comprising: a side frame comprising an outer surface configured to at least partially form an external appearance of the electronic device, a button accommodation recess having a predetermined depth from the outer surface in a first direction oriented to an inner space of the electronic device, and at least one through-hole connected to an inner surface of the side frame in the button accommodation recess; and a key button disposed through the button accommodation recess, the key button comprising a key top inserted into the button accommodation recess and at least partially exposed to an outside of the side frame, and a key base coupled to the key top and comprising a protrusion facing a key flexible printed circuit board, FPCB, disposed in the inner space through the at least one through-hole, wherein the protrusion comprises an engagement portion formed at an end thereof to be larger than the protrusion, and wherein, after the protrusion passes through the at least one through-hole, the engagement portion is engaged with the inner surface around the at least one through-hole, and characterised by a waterproof member disposed between the key FPCB and the inner surface of the side frame to be in close contact therewith, wherein the waterproof member is configured to seal a periphery of the at least one through-hole, and wherein the key FPCB is configured to perform switching operation through which the protrusion and the engagement portion press the waterproof member. a side frame comprising an outer surface configured to at least partially form an external appearance of the electronic device, a button accommodation recess having a predetermined depth from the outer surface in a first direction oriented to an inner space of the electronic device, and at least one through-hole connected to an inner surface of the side frame in the button accommodation recess; and a key button disposed through the button accommodation recess, the key button comprising a key top inserted into the button accommodation recess and at least partially exposed to an outside of the side frame, and a key base coupled to the key top and comprising a protrusion facing a key flexible printed circuit board, FPCB, disposed in the inner space through the at least one through-hole, wherein the protrusion comprises an engagement portion formed at an end thereof to be larger than the protrusion, and wherein, after the protrusion passes through the at least one through-hole, the engagement portion is engaged with the inner surface around the at least one through-hole, and characterised by wherein the protrusion comprises an engagement portion formed at an end thereof to be larger than the protrusion, and wherein, after the protrusion passes through the at least one through-hole, the engagement portion is engaged with the inner surface around the at least one through-hole, and characterised by a waterproof member disposed between the key FPCB and the inner surface of the side frame to be in close contact therewith, wherein the waterproof member is configured to seal a periphery of the at least one through-hole, and wherein the key FPCB is configured to perform switching operation through which the protrusion and the engagement portion press the wherein the waterproof member is configured to seal a periphery of the at least one through-hole, and wherein the key FPCB is configured to perform switching operation through which the protrusion and the engagement portion press the waterproof member.