One object of the present invention is to provide a kneader which can diminish wear of screws and a barrel while preventing the manufacturing process for the screws and barrel from becoming complicated. In this kneader, a pair of screws are rotated about respective axes thereof to knead a material to be kneaded which is introduced into a kneading space formed within a barrel. Each screw has such a shape generating deflection in a predetermined direction perpendicular to an arranged direction of the axes of the pair of screws during kneading of the material to be kneaded, and the kneading space has a shape such that a pair of accommodating spaces which accommodate each of the screws respectively are connected together so as to overlap each other partially in the radial direction, each accommodating space being formed in such a manner that the diameter thereof in the predetermined direction in which there occurs deflection of each screw is larger than in any other direction.