Abstract: An injection molding unit has an injection molding machine 1 for injecting a molten resin plasticized in a screw cylinder 4 from a nozzle 9 fitted at the tip of the screw cylinder 4 by advancing a screw 3. An injection mold 31 forms a hollow 36 for forming an injection molded product 35 with a cavity mold 33 and a core mold 34. The injection molding unit further has a pressure relief valve 40 for releasing the resin out of a resin passage when a pressure greater than a predetermined value acts at any point on the resin passage, including the screw cylinder and ranging up to the terminal end of the hollow. The cavity mold 33 is of a split construction, in which a wall portion of the hollow is formed a plurality of segments.