In a first step, a molding die is immersed in a raw liquid including cellulosic fibers. Then, the liquid of a raw liquid is sucked through a net material by a liquid suction part, so as to cause the cellulosic fibers to be laminated on the net material. In the second step, the cellulosic fibers laminated on the net material are dried, so as to form a protrusion. In the third step, the dried protrusion and the communication part are removed from the molding die. A passage part of the communication part is formed at a position where a projection portion had been arranged, the passage part being configured to communicate an inside of the protrusion with an outside of the protrusion.