To provide a secondary processing method of an injection-molded product capable of simply and inexpensively performing secondary processing without damaging the appearance of a secondary processed product.

In the secondary processing method of the injection-molded product, rubber bands 19 are fitted in the gaps between the ribs 15 provided to the side parts of the branch pipes 12 of an injection-molded branched pipe 10 for a ribbed pipe and the branched pipe 10 is immersed in an oil tank 21 to heat the branch pipes 12 to expand the same in diameter.