To provide a mold for blow molding with excellent durability without generating rust by holding aligning properties of a slide core with a mold body for a long period.

The mold for blow molding comprises a pair of split molds of one mold 11 having a cavity 13 for molding one half of a blow molded molding and the other mold 12 having a cavity 14 for molding the other half of the molding. The mold 12 has a telescopic slide core 16 for welding both resin walls by projecting part of the other resin wall of a hollow double wall structure until it is brought into contact with one resin wall after molds are clamped. Covering layers are formed at an operating site 18 to the resin wall of the core 16 and a slide contact site 19 to be slidably brought into contact with the operating site of the core 16 of the other mold 2.