To provide a cap for the curtain rail whose lateral width is relatively long, wherein light leakage on both sides of a curtain is prevented by shortening a distance where the center of an outermost runner hanging the curtain is separated inward from the end face of the curtain rail when closing the curtain.

A runner groove having the same transverse width as a curtain rail groove and opened on the end face on the opening side is provided on the bottom surface part of the box-shaped body of the cap for the curtain rail, whose end face is opened on one side, wherein the lateral width of the runner groove is set to a half or more of the lateral width of the cap.