PURPOSE: To improve the fitness of a bead part heel while suiting it to airtightness depending on the function of a toe, by making a bead seat part to have a down grade toward an internal angle corner part, inductively keeping a stable seating attitude at the bead part heel, and reinforcing interference at the toe.

CONSTITUTION: A bead seat part 1 and a flange part 2 are provided as a pair so as to received and engage the right and left bead parts of a pneumatic rubber tire, and an internal angle corner part 3, corresponding to the cross section profile curve of a heel of a bead part, is formed. The bead seat part 1 is formed with the rotational surface of an outward recessed curve having a radius of curvature of 20-1000mm forming a reverse tapered form of a down grade toward the internal angle corner part 3, and the angle of the reverse tapered part is made 4-14°. Consequently the bead part heel is made freely loose in a rimming-mounting process to be properly placed to a given place, and the longitudinal shake of a tire is reduced to improve airtightness.