An opening/closing apparatus capable of manipulating a sun visor, which is opened and closed at a front side of a front opening of a helmet main body, from a top portion of the helmet is provided. The opening/closing apparatus includes a sliding member in which its one end portion is coupled to a central top end portion of the sun visor, and a grip helping the sliding member to move forwards and backwards is formed at another end portion, a guide holder that is attached to an inner top portion of the helmet main body and guides a forward and a backward movement of the sliding member, and a damper that is a saw-toothed member and is attached to one side of the guide holder.