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 after the ambient temperature rises from room temperature, when the ambient temperature then falls to room temperature, characterized in that the connector of the union nut thereby radially contracts at a rate higher by 0.05% or more than that of the press-in portion of the inner ring, which enables the connector of the union nut to radially contract greater than the press-in portion of the inner ring, as a result of the fitting meeting one of the following conditions: (a) the inner ring was heat-treated while the union nut was not heat-treated, (b) the inner ring and the union nut were injection molded using different injection pressures, holding pressures, injection rates or mold temperatures, or (c) the resin material of the inner ring is different from the resin material of the union nut. 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 after the ambient temperature rises from room temperature, when the ambient temperature then falls to room temperature, characterized in that the connector of the union nut thereby radially contracts at a rate higher by 0.05% or more than that of the press-in portion of the inner ring, which enables the connector of the union nut to radially contract greater than the press-in portion of the inner ring, as a result of the fitting meeting one of the following conditions: (a) the inner ring was heat-treated while the union nut was not heat-treated, (b) the inner ring and the union nut were injection molded using different injection pressures, holding pressures, injection rates or mold temperatures, or (c) the resin material of the inner ring is different from the resin material of the union nut. the inner ring and the union nut are made of resin, which has a property of contracting after the ambient temperature rises from room temperature, when the ambient temperature then falls to room temperature, characterized in that the connector of the union nut thereby radially contracts at a rate higher by 0.05% or more than that of the press-in portion of the inner ring, which enables the connector of the union nut to radially contract greater than the press-in portion of the inner ring, as a result of the fitting meeting one of the following conditions: (a) the inner ring was heat-treated while the union nut was not heat-treated, (b) the inner ring and (a) the inner ring was heat-treated while the union nut was not heat-treated, (b) the inner ring and the union nut were injection molded using different injection pressures, holding pressures, injection rates or mold temperatures, or (c) the resin material of the inner ring is different from the resin material of the union nut.