A waveguide, that is improved in size, costs, manufacturing and in durability, has a base having a mounting surface, and a plate member held on the mounting surface of the base for defining a waveguide in cooperation with the base. The mounting surface of the base is a curved surface defined by a sweep of a flexure curve of the plate member when it is loaded along a straight line. The plate member is attached to the base in such a manner that the plate member is pressed against and curved along the mounting surface, and that the plate member has a curvature direction edge portion extending in a curvature direction which is a general extension of the flexure curve and a sweep direction edge portion extending in a direction of the sweep of the curve.