A cap includes a bottomed first cover portion, and a second cover portion provided on a receiving opening side of the first cover portion. The second cover portion includes an outside cover that covers a contact portion of a receptacle. The contact portion comes into contact with a distal end of a supply nozzle when the supply nozzle is connected with the receptacle. The outside cover has a tapered shape such that an outer diameter of the outside cover is decreased towards the first cover portion from one end of the cap where the receiving opening is provided. The outside cover has an internal dimension that allows the outside cover to be separated from an outer peripheral portion of the receptacle when the cap is attached to the receptacle.