A tire manufacturing method for vulcanizing a green tire (Tn) using a rigid inner mold comprising an outer surface (s) for molding an inner cavity surface of a pneumatic tire, the method comprising: a step of forming the green tire (Tn) on the outer surface of the rigid inner mold; a vulcanization step of vulcanizing the green tire (Tn) on the rigid inner mold, wherein the rigid inner mold comprises an inner mold body comprising a plurality of segments forming an annular shape with a central hole upon being assembled, a core disposed in the central hole and substantially constraining the respective segments to hold the inner mold body in the annular shape, and an outer diameter adjusting means for changing an outer diameter of the core by changing a temperature of the core such that the respective segments move outward in a radial di recti on of the ti re to increase a gap between adjacent segments (4A, 4B) by an expansion of the outer diameter of the core, and the respective segments move inward in the radial direction of the tire to increase a level difference of outer surfaces between adjacent segments (4A, 4B) by a reduction of the outer diameter of the core, and wherein the vulcanization step comprises an adjustment step of adjusting the outer diameter of the core by the outer diameter adjusting means so that the gap and the level difference fall within predetermined ranges. a step of forming the green tire (Tn) on the outer surface of the rigid inner mold; a vulcanization step of vulcanizing the green tire (Tn) on the rigid inner mold, wherein the rigid inner mold comprises an inner mold body comprising a plurality of segments forming an annular shape with a central hole upon being assembled, a core disposed in the central hole and substantially constraining the respective segments to hold the inner mold body in the annular shape, and an outer diameter adjusting means for changing an outer diameter of the core by changing a temperature of the core such that the respective segments move outward in a radial di recti on of the ti re to increase a gap between adjacent segments (4A, 4B) by an expansion of the outer diameter of the core, and the respective segments move inward in the radial direction of the tire to increase a level difference of outer surfaces between adjacent segments (4A, 4B) by a reduction of the outer diameter of the core, and wherein the vulcanization step comprises an adjustment step of adjusting the outer diameter of the core by the outer diameter adjusting means so that the gap and the level difference fall within predetermined ranges. an inner mold body comprising a plurality of segments forming an annular shape with a central hole upon being assembled, a core disposed in the central hole and substantially constraining the respective segments to hold the inner mold body in the annular shape, and an outer diameter adjusting means for changing an outer diameter of the core by changing a temperature of the core such that the respective segments move outward in a radial di recti on of the ti re to increase a gap between adjacent segments (4A, 4B) by an expansion of the outer diameter of the core, and the respective segments move inward in the radial direction of the tire to increase a level difference of outer surfaces between adjacent segments (4A, 4B) by a reduction of the outer diameter of the core, and wherein the vulcanization step comprises an adjustment step of adjusting the outer diameter of the core by the outer diameter adjusting means so that the gap and the level difference fall within predetermined ranges.