A member joint, in which a great rotating force is inevitably transmitted to a union nut connecting two members at the time of relatively rotating both the members, is structured such as to reliably carry out corotation prevention of the union nut, without complicating fixation of the nut and releasing of the fixation and without causing enlargement in size and deterioration of durability of a connecting portion. In a member joint (10) connecting a first member (1) and a second member (2) by pinching a flange (6) provided in the second member (2) with respect to the first member (1) by a union nut (3), an outer periphery of the first member (1) is provided with a projection (7) having a leading surface (7a) and a trailing surface (7b), a lock ring (4) is rotatably attached to an open leading end portion of the union nut (3), an inner periphery of the lock ring (4) is provided with a lock pawl (9) which can pass over the projection (7) by being elastically deformed, the union nut (3) is provided with a pinching piece (8) which is engaged with the lock pawl (9), and a leading end side of the lock pawl (9) is pinched between the pinching piece (8) and the trailing surface (7b) when the union nut (3) screwed into a screwing end point rotates by itself in a loosening direction, whereby loosening of the union nut (3) is prevented.