PURPOSE: To mass-produce tires free from puncture by a method wherein material is expanded in a rotary die so that the density of a foam reduces as the foamed portion approaches to the outer periphery.

CONSTITUTION: Foaming agent is mixed with rubber or urethane etc., placed in a doughnut-shaped die 6, heated and expanded as it is turned in the direction of the arrow 7. By means of the centrifugal force due to his revolution, the density of the foam reduces as the foamed portion approaches to the outer periphery and especially the highest foam density is obtained in the portion of the tread 8. Thus, the tires having improved wear resistance, and cushioning properties which are larger at places nearer to the inside and free from puncture can be produced at one shot.