To provide a blow molding die which can inhibit a gas leakage during blow molding and blow mold a bellows with high precision.

This blow molding die is divided into a first core mold positioned on the inner peripheral surface side of a large dia. side mounting part, and a second core mold positioned on the inner peripheral surface side of a small dia. side mounting part. In addition, the blow molding die is configured of a core mold with an air jet nozzle for blow molding and a blow mold which blow molds a parison into an end-product shape between this blow mold and the former core mold. Besides, the blow molding die features an annular groove formed in the side surface of the first core mold so as to receive the end part, in the axial direction, of the large dia. side mounting part.