A portable electronic device is provided. The portable electronic device includes a front glass cover that forms the front of the electronic device, a back cover that forms the back of the electronic device, a bezel that surrounds the space formed between the front and back covers and is formed integrally with, or separately from, the back cover, wherein the bezel has a first opening with a first size and a second opening with a second size smaller than the first size, the second opening being disposed adjacent to the first opening, a display device that is located in the space and has a screen area exposed through the front cover, a tray structure that is inserted into the space through the first opening and is detachably mounted, the tray structure having a recess in which a memory card and/or a subscriber identification module (SIM) card is received, a first member that holds the tray structure when the tray structure is inserted into the space and a second member that allows a first member to release the tray structure by the press of an external member that is inserted through the second opening, and a waterproof structure that includes a first portion that is movably inserted into the second opening, a first end portion that makes contact with the second member by the press of the external member, and a second end portion that is pressed by the external member.