A portable electronic device, comprising: a front glass cover forming a front surface of the portable electronic device; a rear cover forming a rear surface of the portable electronic device; a bezel surrounding a space formed between the front cover and the rear cover, formed integrally or separately from the rear cover, and including a portion having an opening; a display device embedded in the space and including a view area exposed through the front cover; and a plate including a flat surface positioned parallel to the front cover in the space, wherein the opening penetrates from an outside of the bezel (20 to the space in a first direction, wherein the portable electronic device further comprises: a key sized and shaped to be able to pass through the opening and including a first surface movably inserted into the opening in the first direction and facing in the first direction, a supporting structure mounted in a portion of the plate or the bezel and including a hole through which the key passes, a dome button mounted in the supporting structure and disposed towards the first surface of the key and the hole to be pressed when the key moves in the first direction, and a waterproof structure including a flexible material, and wherein the waterproof structure includes a first portion positioned between the first surface of the key and the dome button and a second portion extending from the first portion to be inserted between the supporting structure and the bezel and seal to waterproof the space between the supporting structure and the bezel. a front glass cover forming a front surface of the portable electronic device; a rear cover forming a rear surface of the portable electronic device; a bezel surrounding a space formed between the front cover and the rear cover, formed integrally or separately from the rear cover, and including a portion having an opening; a display device embedded in the space and including a view area exposed through the front cover; and a plate including a flat surface positioned parallel to the front cover in the space, wherein the opening penetrates from an outside of the bezel (20 to the space in a first direction, wherein the portable electronic device further comprises: a key sized and shaped to be able to pass through the opening and including a first surface movably inserted into the opening in the first direction and facing in the first direction, a supporting structure mounted in a portion of the plate or the bezel and including a hole through which the key passes, a dome button mounted in the supporting structure and disposed towards the first surface of the key and the hole to be pressed when the key moves in the first direction, and a waterproof structure including a flexible material, and wherein the waterproof structure includes a first portion positioned between the first surface of the key and the dome button and a second portion extending from the first portion to be inserted between the supporting structure and the bezel and seal to waterproof the space between the supporting structure and the bezel. a key sized and shaped to be able to pass through the opening and including a first surface movably inserted into the opening in the first direction and facing in the first direction, a supporting structure mounted in a portion of the plate or the bezel and including a hole through which the key passes, a dome button mounted in the supporting structure and disposed towards the first surface of the key and the hole to be pressed when the key moves in the first direction, and a waterproof structure including a flexible material, and wherein the waterproof structure includes a first portion positioned between the first surface of the key and the dome button and a second portion extending from the first portion to be inserted between the supporting structure and the bezel and seal to waterproof the space between the supporting structure and the bezel.