To provide a joint of a curtain rail, which connects curtain rails so that the relative position in a width direction between a vertical groove of the joint and running grooves of the curtain rails is not shifted to achieve a smooth movement of the runner.

The joint 1 of a curtain rail 2 has a joint top surface portion 9 of elongated rectangular shape, joint side surface portions 10 extending downward from both ends in the width direction of the joint top surface portion 9, and joint bottom surface portions 11 extending inward from the lower ends of the left and right joint side surface portions 10 to form the vertical groove 12. In the joint 1 of the curtain rail 2, the width of the vertical groove 12 is substantially the same as that of the running groove 7 of the curtain rail 2, the joint bottom surface portion 11 overlaps with a rail rolled portion 6, and both ends of the vertical groove 12 are widened in the longitudinal direction, where the joint bottom surface portion 11 near starting points 13 of widened groove, at both ends of the vertical groove 12, is provided with projections 14 projecting toward the side of a rail bottom surface portion 5, and the projection 14 is engaged with the outside edge 6a of the rail rolled portion 6.