The present invention relates to a union device for pipe joints, where the joint is within the wall thickness of the pipes being joined, and includes a female part (1) and a male part (2), which in substantially complemental implementation permits the male part (2) to be inserted into the female part (1). In the outer and inner peripheral surfaces of the male and female parts there are grooves (5,11) extending around the periphery such as to form a cavity for a locking ring when the joint has been assembled. In this cavity there is a locking ring having a rhomboid form in cross section with cut away portions at the long sides of the two diametrically opposite acute corners and being of a material which retains its shape with a given resiliency. This locking ring is provided with a through slit or with one or more shorter slits from its large end towards its small end. The locking ring is normally placed in the groove (5) in the female part (1) with the large end facing axially outwards, such that on inserting the male part (2) into the female part (1) the male part (2) causes the locking ring resiliently to move away, such as to regain its original shape once again when the groove (11) in the male part (2) comes opposite the locking ring.