In a plughole waterproofing device for an engine, a coil case is fitted onto the ignition coil that is accommodated in a plughole of an engine, and an upper portion of the case protruding above the engine is provided with a longitudinal air path extending upward from a lower end thereof communicating with the plughole and a vent hole communicating with an upper end portion of the air path and opened to ambient air. A cover wall hanging while surrounding the vent hole is continuously provided on the case upper portion, and an air chamber is formed inside the cover wall, the vent hole being opened in an upper portion of the air chamber, and an opening face being formed at an entire bottom of the air chamber opening toward the engine.