A measuring device may be provided with a cover configured to close an opening provided on a fuel tank; a plug configured to close a through hole provided on the cover; and a sensor configured to measure a property of fuel inside the fuel tank. The plug may be provided with an electrical connector and a fuel passage. The electrical connector may be configured to electrically connect inside and outside of the fuel tank. The electrical connector may pierce through the plug from an upper surface to a lower surface of the plug The fuel passage may include an inlet configured to open to the inside of the fuel tank to receive the fuel from the inside of the fuel tank. The sensor may be disposed in the fuel passage of the plug.