A resin tube fitting has a body with a sleeve allowing the longitudinal end of the tube to be put therein, an inner ring with a press-in portion configured to be pressed in the longitudinal end of the tube, and a union nut with a connector configured to 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. The inner ring and the union nut are made of resin, which has a property of contracting in response to change in ambient temperature. The radial contraction rate of the connector is higher by 0.05% or more than that of the press-in portion of the inner ring.