NOVELTY - A case (6) has a battery lid (7) that is coupled to a case portion. A rim formed around an opening of a battery compartment (11) pushes and elastically deforms a waterproof rubber seal in an inward radial direction when lid is coupled to case portion. The seal produces an elastic repulsive force when elastically deformed in an inward radial direction.The elastic repulsive force pushes against an inner wall of battery compartment in an outward radial direction to keep battery lid in a state coupled to case portion and to seal a gap between battery lid and case portion in a liquid-tight state.    USE - Portable device for use with communication peer in electronic key system.    ADVANTAGE - When the battery lid is coupled to the case portion, the waterproof element is elastically deformed inward in its radial direction and maintained in this state. The waterproof element is maintained in a state applying outward force to the inner circumferential surface of the annular step. The waterproof element seals the gap between the battery lid and case portion in a liquid-tight state. Thus the need to fasten the battery lid with a screw to the case portion is eliminated. Therefore, there is no need for a fastening structure used to compress a waterproof element so that it becomes waterproof. The battery lid is coupled to the case portion just by forcing the battery lid toward the case portion, and the waterproof element seals the battery lid and the case portion in a liquid-tight state. This eliminates the need for performing fastening with a screw. The simple structured flexible groove extends throughout the entire annular waterproof element. The flexible groove facilitates deformation of the waterproof element in the radial direction, thereby facilitating the production of a mold used to integrally mold the waterproof element and the battery lid. The silicone rubber functions to prevent slipping when the user holds the battery lid in addition to functioning as a stimulus to the user when touched. The user is allowed to determine that the waterproof element is in a waterproof state. The elastic repulsive force of the waterproof element maintains the battery lid in a state coupled to the case portion. Since a structure for fastening with a screw can be eliminated, the structure of the electronic key is simplified. This allows for reduction in the size of the case portion.    DESCRIPTION OF DRAWING(S) - The drawing shows an exploded perspective view showing the electronic key.    Case (6)    Battery lid (7)    Cover (8)    Battery (9)    Battery compartment (11)