A union joint capable of connecting two pipes in a perfectly sealed joint. The union joint comprises a first sleeve hermetically joined to a first pipe, a second sleeve hermetically joined to a second pipe, a metallic gasket interposed between the first and second sleeves so as to be compressed between the respective pressing surfaces of the first and second sleeves when the first and second sleeves are drawn axially toward each other and a coupling nut combined with the first sleeve so as to be screwed on the second sleeve to draw the first and second sleeves axially toward each other. Grooves having a shape substantially conforming to the external shape of the metallic gasket are formed respectively in the respective end surfaces of the first and second sleeves, a retaining member for retaining the metallic gasket in place is detachably put on either the first or second sleeve, and the retaining member has a limiting portion which engages the end surfaces of the first and second sleeves to limit the compressin of the metallic gasket when the first and second sleeves are drawn axially toward each other. The retaining member holds the metallic gasket in place in assembling the union joint to facilitate assembling the union joint. The bottom surfaces of the grooves can be easily burnished to secure perfect sealing contact between the bottom surfaces of the grooves and the metallic gasket.