When a molding machine nozzle is separated from a nozzle touch surface of a hot runner sprue, application of external pressure on resin in a resin supply passage is stopped. At this time, back pressure of resin in the resin supply passage that is received by a seal exceeds atmospheric pressure, and thus the seal is moved to seal an opening of the resin supply passage from the inside of the resin supply passage. As a result, leakage of resin from the hot runner sprue is inhibited, and mixing of air in the resin in a runner is also inhibited.