A blow molding method by horizontal blow molding of piercing the wall of a parison having been extruded by an extruder with a blow nozzle is disclosed, which comprises moving forward the blow nozzle to pierce the parison, introducing blow air at least in the direction toward the bottom, i.e., opposite to the bottle neck and, after shaping of a hollow container by the blow air, forming an opening in the parison at a position in the vicinity of or above the blow nozzle-pierced portion to discharge the blow air having circulated inside the hollow container. In this blow mollding method, there is preferably used a blow air cylinder, which is constituted by (A) a blow nozzle having a mechanism for registering blowing inlets in at least part of a portion freely sliding forward and backward inside the body, having a tip portion with an outer diameter smaller than at about the center portion thereof and being of a shape of a closed injection needle, and having the blowng inlets located slightly at the back of the tip portion where the outer diameter is larger than that at the tip portion and smaller than that at the central portion and capable of introducng blow air in the direction toward the bottom, i.e., opposite to the bottle neck in an amount of 30% or more of the total blow air amount; and (B) a body provided outside and, at least partly, in cntact with the blow nozzle.