PURPOSE: To enhance strength of a weld by a method wherein fusion weld is conducted while twisting one of pipes about its axis during application of a pressure and then the weld is cooled, in the case where end faces of synthetic pipes are abutted against each other and fusion welded under pressure after the faces are molten by by heating.

CONSTITUTION: After end faces of synthetic resin pipes 1 and 2 to be fusion welded are molten by heating by heater plates, the plates are removed, and the end faces are abutted against each other and fusion welded under pressure. During application of the pressure, one pipe 1 is fixed, the other 2 is applied with the pressure in the direction of an arrow b for fusion welding while being twisted about its axis as indicated by an arrow a, and the weld is cooled. By this, a shearing stress in torsion is applied to the weld interface, which enhances the interaction between the resins and hence the strength of the weld A.