To readily manage small-diameter air tubes between air devices in aquariums and an air supply tube on the outside of the aquariums in good appearance and supply air from an air supply tube to the two air devices by enabling the two air devices in the aquariums to be connected through small- diameter air tubes to the air supply tube.

This joint is obtained by respectively installing a single first connecting port P1 at one end 1a of the joint body 1 and a pair of second connecting ports P2 and P3 formed into a smaller diameter than that of the first connecting port P1 and mutually parallel at the other end 1b and forming the other end 1b of the joint body 1 into a flat shape in the direction intersecting the parallel direction of the second connecting ports P2 and P3 at right angles. An air supply tube Ap can be connected to the first connecting port P1 and flexible air tubes C1 and C2 connected to a pair of air devices F and S and having a smaller diameter than that of the air supply tube Ap can be connected to the pair of the second connecting ports P2 and P3.