<p>The present invention addresses the technical problem of effectively minimizing the incidence of stretch marks during blow molding using a liquid as a pressure medium, without dramatically changing the configuration of the device. In a blow molding device having a mold for blow molding and a blow nozzle that is tightly linked to a nozzle part of a cup-shaped preform, and where a liquid is provided as a pressure medium in the preform through this blow nozzle, the blow nozzle has an insertion tube piece that inserts into the nozzle part, the blow nozzle extending from a lower end part. The insertion tube piece extends by such a length that a lower end of the insertion tube piece extends below a neck ring provided around a lower end of the nozzle part.</p>
