Abstract: A parison length controller for molding parisons having a predetermined length and a uniform wall thickness distribution over the whole length. A parison lower edge detector is provided for generating the parison lower edge signal when a parison is extruded to a predetermined length, and a parison controller generates the extrusion end signal every time when an extrusion plunger reaches the end of the extrusion stroke. Depending upon whether the parison lower edge signal or the extrusion end signal occurs first, a positive or negative voltage signal of a predetermined magnitude is generated, and the positive and negative voltage signals are accumulated and stored and used as a wall thickness correction signal for forming a parison having a predetermined length.