A conveyance portion, a barrier portion, and a path are provided at places of a portion of a screw main body in which a kneading portion is provided. In at least one of the places, an entrance is opened to cause raw materials, conveyance of which is limited by a barrier portion to increase pressure on the raw materials, to flow in. The raw materials flowing in from the entrance flow through the path in the opposite direction to a conveyance direction of the conveyance portion. An exit is opened in an outer circumferential surface of the screw main body at a position outside the conveyance portion in which the entrance is opened.