In order to stretch an injection-molded preform by a stretching rod in the axis direction and to blow mold it into a hollow product such as a vessel with a predetermined shape in a blow mold, some of the blow air after used is collected in a low-pressure working-air tank set separately from a high-pressure blow-air tank and operating selector valves in a circuit and the collected air is used for some of the working air for an air cylinder for operating units related to stretch-blow molding by air cylinders. To decrease the availability factor of a compressor by collecting used blow air and thereby, make it possible to cut the cost required for compressed air and decrease exhaust noises.