A pipe detachment preventing device for a union nut pipe fitting, comprising: a fitting body having a receiving portion which has an external thread formed on an outer peripheral surface thereof and into which a non-threaded joint pipe is inserted; a union nut screwed and tightened to the receiving portion with an annular rubber packing interposed therebetween; a retaining ring fixed to an outer peripheral surface of the joint pipe by reducing a diameter of a retaining ring body by tightening means; a locking tool detachably attached to the union nut; and connection holding means configured to hold connection between the retaining ring and the locking tool, wherein either the retaining ring includes a pair of first protruding portions protruding outward from the retaining ring body, and a blade portion which is provided on an inner side of the retaining ring body and which bites into the joint pipe due to diameter reduction by the tightening means, and the locking tool includes a pair of first facing portions which are provided at a pair of arm portions extending along an axial direction from a main body and which face the first protruding portions in a pipe axis direction, or the locking tool includes a pair of second protruding portions protruding inward at end portions of a pair of arm portions extending along a pipe axis direction from a main body, and the retaining ring includes a pair of second facing portions which are provided on an outer peripheral surface of the retaining ring body and which face the second protruding portions, and a blade portion which is provided on an inner side of the retaining ring body and which bites into the joint pipe due to diameter reduction by the tightening means, and the connection holding means includes an inclination mechanism configured to incline the retaining ring in the pipe axis direction with respect to a reference plane orthogonal to a central axis of the retaining ring body, such that one of a circumferential center portion or a circumferential end portion of the retaining ring body is caused to be closer to the fitting body and the other is moved away from the fitting body, when either the first protruding portions and the first facing portions or the second protruding portions and the second facing portions come into contact with each other and a pipe pulling force which stretches the joint pipe is applied to the joint pipe. a fitting body having a receiving portion which has an external thread formed on an outer peripheral surface thereof and into which a non-threaded joint pipe is inserted; a union nut screwed and tightened to the receiving portion with an annular rubber packing interposed therebetween; a retaining ring fixed to an outer peripheral surface of the joint pipe by reducing a diameter of a retaining ring body by tightening means; a locking tool detachably attached to the union nut; and connection holding means configured to hold connection between the retaining ring and the locking tool, wherein either the retaining ring includes a pair of first protruding portions protruding outward from the retaining ring body, and a blade portion which is provided on an inner side of the retaining ring body and which bites into the joint pipe due to diameter reduction by the tightening means, and the locking tool includes a pair of first facing portions which are provided at a pair of arm portions extending along an axial direction from a main body and which face the first protruding portions in a pipe axis direction, or the locking tool includes a pair of second protruding portions protruding inward at end portions of a pair of arm portions extending along a pipe axis direction from a main body, and the retaining ring includes a pair of second facing portions which are provided on an outer peripheral surface of the retaining ring body and which face the second protruding portions, and a blade portion which is provided on an inner side of the retaining ring body and which bites into the joint pipe due to diameter reduction by the tightening means, and the connection holding means includes an inclination mechanism configured to incline the retaining ring in the pipe axis direction with respect to a reference plane orthogonal to a central axis of the retaining ring body, such that one of a circumferential center portion or a circumferential end portion of the retaining ring body is caused to be closer to the fitting body and the other is moved away from the fitting body, when either the first protruding portions and the first facing portions or the second protruding portions and the second facing portions come into contact with each other and a pipe pulling force which stretches the joint pipe is applied to the joint pipe.