Resin pipe fitting

A resin tube fitting connectable to a tube, while allowing a longitudinal end of the tube to be put therein, the fitting comprising:

• a body including a sleeve that allows the longitudinal end of the tube to be put therein;
• an inner ring including a press-in portion that is configured to be pressed into the longitudinal end of the tube, and to be put inside the sleeve together with the longitudinal end of the tube in which the press-in portion is put;
• and
• a union nut including a connector that is configured to, aiming at connecting the longitudinal end of the tube to the body, be screwed to an outer periphery of the body to allow the longitudinal end of the tube to be put radially between the connector and the press-in portion of the inner ring when the press-in portion is put inside the sleeve of the body together with the longitudinal end of the tube, wherein:
• the inner ring and the union nut are made of resin, which has a property of contracting when ambient temperature rises from room temperature and then falls to room temperature;
• and
• the connector of the union nut is designed to radially contract at a rate higher by 0.05% or more than the press-in portion of the inner ring when ambient temperature rises from room temperature and then falls to room temperature.