In a method for forming a green tread rubber comprised of a cap rubber layer which outer surface forms a tread surface and a base rubber layer that adjoins the same inside thereof in the radial direction, a cap rubber having a rubber component of styrene/butadiene rubber or being a mixed rubber of natural rubber and butadiene rubber is used as the cap rubber layer. The base rubber layer is formed as an annular body in which a strap-like rubber extruded body of wide width, which is extruded from a rubber extruder and is cut into constant size, is wound around a molding drum by a single round with their end portions in a circumferential direction being joined with each other. The cap rubber layer is formed as a strip layered body in which a long strap-like rubber strip of narrow width, which is extruded from an rubber extruder, is successively wound by overlapping the same on the annular body in the circumferential direction and in a spiral manner. A thickness T2 of the base rubber layer on a tire equator is defined to be 0.05 to 0.7 times a thickness T0 of the green tread rubber on the tire equator.