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 . <EOS>