Disclosed is a vulcanizing method of a pneumatic bias tire having an aspect ratio Y of 25% or more and 55% or less from an unvulcanized tire, which comprises the step of expanding a rubber-made bladder from an inside of the unvulcanized tire in metallic molds, in which a bladder stretch ratio X of an outer diameter of the bladder when it is not expanded relative to a tire inner surface diameter, when the unvulcanized tire is pressed to the inner surfaces of the metallic molds by means of expansion of the bladder, is set in a range expressed by the following expression: X=&bgr;−Y/&agr;, where 0.37