PURPOSE: To prevent a synthetic resin pipe from its shear crack by contracting an end part external diameter of a stiffener provided in the axial inward side of the synthetic resin-made pipe, in a connecting structure for the synthetic resin-made pipe relating to a metal-made connecting cylinder.

CONSTITUTION: A stiffener 7 is constituted of a flange part 15 opposed to an end face of a pipe 3, straight cylinder part 16 advanced into the pipe 3 and an end part 17 of changing the external diameter into small size toward an axial inward side of the pipe 3. This metal-made stiffener 7 is inserted into the pipe 3 of polyethylene or the like inserted into a connecting cylinder 2 of 4-1, to spread a diameter of the pipe 3, and an external surface of the pipe 3 is closely attached to an O ring 9, provided in an internal surface of the connecting cylinder, to ensure air-tightness. Here, since an external diameter in the end part 17 of the stiffener 7 is changed into small size, even when the pipe 3 is deflected as shown by a virtual line 18, an internal surface of the pipe 3 is smoothly guided by the end part 17, to prevent damage without receiving action of shearing force.