A vent passage forming structure includes a penetration hole portion, a groove portion and a vent passage forming plate. The penetration hole portion is formed on a housing of an earphone. A space on one side of the housing communicates with a space on the other side of the housing through the penetration hole portion. The groove portion is formed on a surface of the housing on the one side thereof to be connected with the penetration hole portion. The vent passage forming plate covers the penetration hole portion and a part of the groove portion. A vent passage through which the space on the one side communicates with space on the other side, is formed by a hole space that is an inner space of the penetration hole portion, and a groove space that is surrounded by the groove portion and the vent passage forming plate.