In a resin pipe joint having a configuration where, by a screw advancement of a union nut caused by screwing an internal thread to an external thread in a state where a tube is externally fitted to an inner cylinder to form a flared portion, a flaring changing region is pressed by a seal pressing portion, a torque varying portion in which projecting portions of a joint body and recess portions of the union nut are fitted to or unfitted from each other by a flexural displacements of the projecting portions in the direction of the axis in accordance with fastening rotation of the union nut from the vicinity of a portion in front of an end of the screw advancement, the screw advancement being caused by pressing of the seal pressing portion against the flaring changing region is configured.