To keep the expected product dimension of a reducer made of an olefinic resin over a long period of time without adding an annealing process to a manufacturing process.

A method for producing a reducer made of an olefinic resin having the small caliber part continued from a large caliber part includes a process for heating the predetermined length part from one end of a material pipe 1 made of an olefinic resin to the vicinity of the m.p. of the material pipe 1 and a process for forcibly inserting the heated material pipe 1 in a cooled mold having a large diameter part 9, a taper part 10 and a small diameter part 11 continuously formed to the inner surface thereof and quenching the interior of the material pipe 1 while contracting the diameter of the heated part of the material pipe 1. An apparatus adapted to this method consists of the above mentioned mold 4, a heating device of the small diameter part 11 of the mold, an insertion device 14 for inserting the material pipe in the molds and cooling devices 7, 8, 15 of the material pipe in the mold.