A resin tube fitting connectable to a tube, comprising: a body including an outer sleeve; an inner ring including an insert portion insertable into the outer sleeve to contact the outer sleeve radially, and a press-in portion configured to be pressed into a longitudinal end of the tube and a union nut configured to be engageable with the body to maintain the insert portion within the outer sleeve, wherein: the body and the inner ring 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 outer sleeve of the body thereby radially contracts at a rate higher by 0.09% or more than the insert portion of the inner ring, which enables the outer sleeve of the body to contract greater than the insert 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 body was not heat-treated, (b) the body and the inner ring were injection molded using different injection pressures, holding pressures, injection rates or mold temperatures, or (c) the resin material of the body is different from the resin material of the inner ring. a body including an outer sleeve; an inner ring including an insert portion insertable into the outer sleeve to contact the outer sleeve radially, and a press-in portion configured to be pressed into a longitudinal end of the tube and a union nut configured to be engageable with the body to maintain the insert portion within the outer sleeve, wherein: the body and the inner ring 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 outer sleeve of the body thereby radially contracts at a rate higher by 0.09% or more than the insert portion of the inner ring, which enables the outer sleeve of the body to contract greater than the insert 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 body was not heat-treated, (b) the body and the inner ring were injection molded using different injection pressures, holding pressures, injection rates or mold temperatures, or (c) the resin material of the body is different from the resin material of the inner ring. the body and the inner ring 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 outer sleeve of the body thereby radially contracts at a rate higher by 0.09% or more than the insert portion of the inner ring, which enables the outer sleeve of the body to contract greater than the insert 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 body was not heat-treated, (b) the body and the inner (a) the inner ring was heat-treated while the body was not heat-treated, (b) the body and the inner ring were injection molded using different injection pressures, holding pressures, injection rates or mold temperatures, or (c) the resin material of the body is different from the resin material of the inner ring.