To provide a joint of a synthetic resin used for supplying water or hot water, which is easily disengaged from a threaded hole even if it is cut during screwing.

A joint A comprises a synthetic resin joint body and a flow path 3 that penetrates it. A screw is provided on both ends of the through flow path 3, and a rotary engagement part is provided on the outside surface of the joint body. The synthetic resin joint has a locking part 3c formed so that a rotary tool engages with the through flow path. With this configuration, even if the synthetic resin joint is cut by an excessive twisting force, it is easily disengaged from the opponent threaded part.