A blow molding mold for forming a resin container by blow-molding a preform, the blow molding mold comprising: a neck mold (23, 501) including a plurality of neck split molds and configured to grip an outer peripheral surface of a neck portion of the preform; a blow cavity mold including a plurality of blow split molds and configured to form an outer peripheral surface of a body portion of the resin container; and a blow core mold unit configured to close an opening of the neck portion of the preform and having a first supply flow path for supplying a high-pressure fluid into the preform, wherein the blow core mold unit includes a first core member including a main body having an outer diameter smaller than an inner diameter of the neck portion of the preform and a loosely fitted portion provided at a tip end portion of the main body and configured to be loosely fitted to the neck portion of the preform, and a second core member provided on an outer peripheral portion of the main body and in contact with the neck mold (23, 501), characterized in that the blow core mold unit includes a third core member having a ring shape, provided in a space portion formed between an outer peripheral surface of the main body and an inner peripheral surface of the second core member so as to be slidable with respect to an outer peripheral surface of the first core member and the inner peripheral surface of the second core member, and configured to contact with a top surface of the neck portion of the preform, and the blow core mold unit has a second supply flow path for supplying the high-pressure fluid into the space portion. a neck mold (23, 501) including a plurality of neck split molds and configured to grip an outer peripheral surface of a neck portion of the preform; a blow cavity mold including a plurality of blow split molds and configured to form an outer peripheral surface of a body portion of the resin container; and a blow core mold unit configured to close an opening of the neck portion of the preform and having a first supply flow path for supplying a high-pressure fluid into the preform, wherein the blow core mold unit includes a first core member including a main body having an outer diameter smaller than an inner diameter of the neck portion of the preform and a loosely fitted portion provided at a tip end portion of the main body and configured to be loosely fitted to the neck portion of the preform, and a second core member provided on an outer peripheral portion of the main body and in contact with the neck mold (23, 501), a first core member including a main body having an outer diameter smaller than an inner diameter of the neck portion of the preform and a loosely fitted portion provided at a tip end portion of the main body and configured to be loosely fitted to the neck portion of the preform, and a second core member provided on an outer peripheral portion of the main body and in contact with the neck mold (23, 501), characterized in that the blow core mold unit includes a third core member having a ring shape, provided in a space portion formed between an outer peripheral surface of the main body and an inner peripheral surface of the second core member so as to be slidable with respect to an outer peripheral surface of the first core member and the inner peripheral surface of the second core member, and configured to contact with a top surface of the neck portion of the preform, and the blow core mold unit has a second supply flow path for supplying the high-pressure fluid into the space portion.