An example portable electronic device includes a front glass cover; a rear cover; a bezel surrounding a space defined by the front cover and the rear cover, the bezel including a portion including an opening; a display device built in the space; and a plate including a plane parallel to the front cover within the space. The example electronic device includes a key movably inserted in the opening; a support structure mounted on the plate or a portion of the bezel and including a hole through which the key passes; a dome button mounted on the support structure and arranged to face the first surface of the key and the hole so that the key can be pressed; and a waterproof structure including a flexible material.