A portable electronic key easily findable even if dropped into water. The portable electronic key includes an upper cover and a lower cover, a hermetically sealed space is formed, in which electrical equipment such as a board, a battery, and an antenna are arranged. The upper cover and the lower cover are formed of foam resin, and a grain pattern is given to surfaces thereof. A specific gravity of the entire electronic key is set at 1 or less, thus enabling the portable electronic key to float on the surface of the water.