Abstract: There is provided a plunger position detector for detecting a linear position of a plunger used for forming a parison over the entire range of moving of the plunger. Since there is correlation between the plunger position and the gob weight, the gob weight can be detected in response to an output signal of this detector and, in accordance with this detection, a gob forming operation is controlled to an optimum state so that the gob weight will become a predetermined weight. Since difference in center positions between a blank mold for forming a parison and the plunger causes variation in the state of movement of the plunger, this difference in the center positions can be detected in response to a plunger position detection signal and, in accordance with this detection, an optimum registering of the center positions can be performed.