To exactly position a preform by finely adjusting its position in the circumferential direction before it is stretched while it is positioned by using a non-contact type sensor.

The perform is rotated to provisionally decide the position in the circumferential direction of a detection shape in a prescribed shape in relation to a blow-molding mold, by detecting the detection shape set at a prescribed position in the circumferential direction of the preform by the non- contact type sensor between a preform heating process and a blow process by a blow mold. After that, by clamping the blow molding mold for the preform, a positioning mold surface formed in the blow molding mold is engaged with the detection shape. In this way, the position in the circumferential direction of the detection shape in relation to the blow molding mold is fixed after being positioned finally.