Abstract: A method of thickening a peripheral portion of a circular plate blank, wherein the blank is held on a jig with its peripheral portion extending radially outwardly from the outer circumferential surface of the jig, and rotated such that the outer circumferential surface of the blank is held in pressing contact with the bottom surface of an annular forming groove formed in a rotatably supported roller die which has first and second spaced-apart side surfaces connected to each other by the bottom surface, the first side surface being inclined with respect to the radial direction of the blank holder jig such that the axial distance between the two side surfaces increases in the radially outward direction, and wherein the force between the blank and the bottom surface of the groove is controlled so that the peripheral portion is held flexed in a convex shape following the first side surface and so that the end part of the peripheral portion is thickened in opposite directions toward the two side surfaces while the