A box-shaped enclosure of a storage box includes a deadening wall or walls defining a storage space between first and second planes. The storage space is open at the first and second planes. A deadening wall member extends along the first plane. An auxiliary box-shaped enclosure is connected to the box-shaped enclosure so as to close the first plane of the box-shaped enclosure. The auxiliary box-shaped enclosure defines an auxiliary space isolated from the storage space with a deadening wall member. The storage space is connected to the fresh air through the ventilation opening and the auxiliary space. The deadening wall or walls of the box-shaped enclosure and the deadening wall member serve to prevent the leakage of the operating sound.