PURPOSE: To easily perform molding by performing bending processing and diameter expanding and contracting processing during a period incomplete in crystal transfer immediately after extrusion molding and capable of performing processing without leaving internal stress to mount a polybutylene pipe on a jig to hold the pipe until the completion of crystal transfer.

CONSTITUTION: A polybutylene pipe 1A subjected to extrusion molding is easily subjected to bending processing or diameter expanding or contracting processing during a period incomplete in crystal transfer immediately after molding and capable of performing processing without leaving internal stress, that is, within two days at environmental temp. of about 33°C. Thereafter, the pipe is held until crystal transfer is completed to obtain a molded product. Therefore, even piping having a small bending radius of curvature or profile piping can be easily molded and piping space can be made compact.