A closing cover integrally includes a cover main body, and a ventilation cap body. The ventilation cap body has a connecting portion that is integrally connected to the cover main body, and at least one ventilation passage hole that is disposed in the connecting portion to perpendicularly extend, and has a depth smaller than a plate thickness of the cover main body. A ventilation recessed portion is formed at a location that is positioned on an inner surface of the ventilation cap body of the closing cover. An inner wall surface defining the ventilation recessed portion is formed in such a shape that a deep side of the ventilation passage hole is opened to the inner wall surface defining the ventilation recessed portion to form a ventilation passage. A ventilation filter is installed on an inner opening portion of the ventilation recessed portion.