A method of manufacturing an engine valve intermediate product with a boss portion at which a valve head forming portion consisting of a neck forming portion that is formed so as to increase in diameter in concave fashion as one proceeds from a base end portion which is continuous with the stem forming portion toward a tip end portion, and a face forming portion that is continuous with the tip end portion of the neck forming portion, are present in integral fashion at a tip end portion of the stem forming portion, and at which the boss portion, an outside diameter of which is greater than that of the stem forming portion, is formed in integral fashion at an outer circumferential surface at an intermediate location at the stem forming portion, the method of manufacturing the engine valve intermediate product with the boss portion being characterized in that it comprises: a stem retaining operation in which the face forming portion, an outside diameter of which is greater than that of the stem forming portion of the intermediate product, is inserted in and retained by a retaining hole in such fashion as to cause a bottom face of the valve head forming portion of the intermediate product to come in contact with a bottom of a valve head jig which is in a shape of a cylinder that is closed at one end, an intermediate location of the stem forming portion is retained by a plurality of intermediate jigs (6a, 6b, 6c) capable of being removably installed radially with respect to a center of the stem forming portion of the intermediate product, the valve head jig and intermediate jigs (6a, 6b, 6c) are retained by tip end jig which is in the shape of a cylinder that is closed at one end; and the base end portion of the stem forming portion is retained by a base end jig; and a stem enlarging operation in which compressive forces are applied from both ends of the intermediate product and the boss portion is formed as the stem forming portion of the intermediate product is made to rotate while it is bent in angular fashion in the vicinity of base end faces of the intermediate jigs (6a, 6b, 6c) and alternating loads that cause tensile forces and compressive forces to act in repeated and alternating fashion in a direction parallel to a central axis of the stem at the outer circumferential surface of the stem forming portion are applied thereto. a stem retaining operation in which the face forming portion, an outside diameter of which is greater than that of the stem forming portion of the intermediate product, is inserted in and retained by a retaining hole in such fashion as to cause a bottom face of the valve head forming portion of the intermediate product to come in contact with a bottom of a valve head jig which is in a shape of a cylinder that is closed at one end, an intermediate location of the stem forming portion is retained by a plurality of intermediate jigs (6a, 6b, 6c) capable of being removably installed radially with respect to a center of the stem forming portion of the intermediate product, the valve head jig and intermediate jigs (6a, 6b, 6c) are retained by tip end jig which is in the shape of a cylinder that is closed at one end; and the base end portion of the stem forming portion is retained by a base end jig; and a stem enlarging operation in which compressive forces are applied from both ends of the intermediate product and the boss portion is formed as the stem forming portion of the intermediate product is made to rotate while it is bent in angular fashion in the vicinity of base end faces of the intermediate jigs (6a, 6b, 6c) and alternating loads that cause tensile forces and compressive forces to act in repeated and alternating fashion in a direction parallel to a central axis of the stem at the outer circumferential surface of the stem forming portion are applied thereto.