Abstract: Disclosed is a structure for preventing a leak at a synthetic resin tube joint, which includes a synthetic resin tube including a tapered portion at an end portion; a ferrule inserted into the end portion of the synthetic resin tube and including a tapered portion corresponding to the tapered portion of the synthetic resin tube; a fitting which accommodates the synthetic resin tube and the ferrule, and includes a tapered portion corresponding to the tapered portion of the synthetic resin tube and a protrusion formed on the tapered portion of the fitting, and a nut which engages with the fitting.