A method of manufacturing a boot for a constant-velocity universal joint, including a large-diameter side end portion into which a casing of a tripod joint including a plurality of concave portions in an outer peripheral surface is inserted, a small-diameter side end portion into which a shaft portion connected to the tripod joint is inserted, and a bellows portion formed between the large-diameter side end portion and the small-diameter side end portion and constituted of repeatedly arranged large-diameter portions and small-diameter portions, the large-diameter side end portion including an inner peripheral surface constituted of a plurality of thick portions adapted to the concave portions of the casing of the tripod joint and protruded on an inner-diameter side and a plurality of thin portions arranged between the thick portions, the method comprising the steps of: holding a primary molded member molded including the small-diameter side end portion and the large-diameter side end portion on opposite ends communicating with an inner space of the bellows portion in divided molds, disposing a core mold in the large-diameter side end portion of the primary molded member, and holding the small-diameter portion just in the vicinity of the large-diameter side end portion by a protrusion formed on an inner peripheral surface of the divided molds and a concave peripheral groove formed in a core mold outer peripheral surface to form a secondary molding space (43, 43a-e) constituted of the thick portions and the thin portions formed in either or both of the inner peripheral surface and the outer peripheral surface of the large-diameter side end portion between a large-diameter side end portion inner peripheral surface of the primary molded member and a core mold outer peripheral surface, between the divided molds inner peripheral surface and the large-diameter side end portion outer peripheral surface of the primary molded member, or between the core mold outer peripheral surface and the divided molds inner peripheral surface via the inner/outer peripheral surface of the large-diameter side end portion of the primary molded member; positioning a molten material injection point for secondary molding in one or a plurality of optional places in the secondary molding space (43, 43a-e), and injecting a molten material into the secondary molding space (43, 43a-e) via the injection point to mold a secondary molded portion on either or both of the inner peripheral surface and the outer peripheral surface of the large-diameter side end portion of the primary molded member; and integrally molding a secondary molded portion on either or both of the inner peripheral surface and the outer peripheral surface of the large-diameter side end portion of the primary molded member to form the boot, moving at least a part of the core mold outer peripheral surface which abuts on an end portion of the thick portion in the vicinity of the bellows portion in a central radial direction of the core mold to remove the part from a thick portion region just before separating the boot from the core mold, and separating the core mold from the boot in this state. holding a primary molded member molded including the small-diameter side end portion and the large-diameter side end portion on opposite ends communicating with an inner space of the bellows portion in divided molds, disposing a core mold in the large-diameter side end portion of the primary molded member, and holding the small-diameter portion just in the vicinity of the large-diameter side end portion by a protrusion formed on an inner peripheral surface of the divided molds and a concave peripheral groove formed in a core mold outer peripheral surface to form a secondary molding space (43, 43a-e) constituted of the thick portions and the thin portions formed in either or both of the inner peripheral surface and the outer peripheral surface of the large-diameter side end portion between a large-diameter side end portion inner peripheral surface of the primary molded member and a core mold outer peripheral surface, between the divided molds inner peripheral surface and the large-diameter side end portion outer peripheral surface of the primary molded member, or between the core mold outer peripheral surface and the divided molds inner peripheral surface via the inner/outer peripheral surface of the large-diameter side end portion of the primary molded member; positioning a molten material injection point for secondary molding in one or a plurality of optional places in the secondary molding space (43, 43a-e), and injecting a molten material into the secondary molding space (43, 43a-e) via the injection point to mold a secondary molded portion on either or both of the inner peripheral surface and the outer peripheral surface of the large-diameter side end portion of the primary molded member; and integrally molding a secondary molded portion on either or both of the inner peripheral surface and the outer peripheral surface of the large-diameter side end portion of the primary molded member to form the boot, moving at least a part of the core mold outer peripheral surface which abuts on an end portion of the thick portion in the vicinity of the bellows portion in a central radial direction of the core mold to remove the part from a thick portion region just before separating the boot from the core mold, and separating the core mold from the boot in this state.