PURPOSE: To manufacture a partition with stabilized dimension, shape and quality of ribs at high speed by forming an outer diameter of an end of a nozzle smaller than the channel width of an outer peripheral section of a molding channel and positioning the end of the nozzle close to the center of the molding channel or slightly getting therein.

CONSTITUTION: A gun nozzle 3 is fixed in a manner that respective couples of nozzles 4 correspond with in the center of respective ring molding channels 1, and molten resin is discharged from a position close by or a position slightly getting into ring molding channels 1 corresponding with respective nozzles 4. Outer diameters D of discharge outlets of the ends of respective nozzles 4 are formed larger than the discharge outlet diameter (d) of an opening 5 with a step and smaller than the channel width W of the outer peripheral sec tion of the molding channel 1. As resin is not sealed and opened to atmosphere at the time of filling resin to the molding channel 1, air is not drawn in, and foams are scarcely generated in the filled resin, and also resin is fed in compli ance with the volume of the molding channel 1 to lessen the generation of flashes and molding resin ribs continuously at high speed.