A tire building apparatus comprises a group of many segments enlarging to pushingly lock a bead portion of a green tire, a ring-shaped inner piston engaging with each segment of such a group, a ring-shaped outer piston contacting and engaging with a back face of the inner piston, a cylinder slidably housing the inner piston and the outer piston therein, a pair of gas pressure acting means moving each of the inner piston and outer piston inward or outward in the direction of the axial line, and a stopper formed in the cylinder.