To provide a rail end cap in which a runner can be easily detached and installed to a curtain rail, which prevents the runner from coming off in a normal use, can fully close a curtain, can be adopted to a large scale curtain for a factory.

A rail end cap has a pair of right and left side plates 5 having holding piece part 10 holding curtain rails 2, extending piece parts 11 extending in an extending direction of the curtain rail 2, and an oscillating flap 7 housed between the side plates 5 and held such that an end part closer to the curtain rail 2 can oscillate up and down around the oscillating shaft 31, and a spacer plate 6 coupling at least the extending piece parts 11 of the side plates 5 each other in a width direction, the oscillating flap 7 has a pair of right and left rail parts 30 oppositely arranged while forming a runner housing groove 34 from an end closer to the curtain rail 2 to an oscillating shaft 31.