A flexible joint including: a socket including a front end portion provided with O-ring grooves, and a male threaded portion extending outward radially, circumferentially, and stepwisely from the front end portion with male threads formed thereon; a nut including an engaging portion engaged with an outer circumferential surface of the front end portion, and a female threaded portion extending stepwisely from the engaging portion and engaged with the male threaded portion; a sprinkler flexible tube communicating with an inner circumference of the socket, and having an enlarged portion inserted between the front end portion of the socket and the engaging portion of the nut, and a leak-proof portion or a double bent portion bent outward from an end of the enlarged portion; and O-rings fitted over the O-ring grooves, whereby the enlarged portion presses the O-rings to come into close contact with each other, thereby preventing leakage.