An umbrella has a central shaft, a crown, a runner, a plurality of ribs, and a plurality of stretchers. The crown is a polygon each side of which includes a pivot pin mounted in a perpendicular direction to pivotally connect the upper end of a respective one of the ribs to the crown. The runner is a polygon and has a pivot pin mounted to each side in perpendicular direction to pivotally connect the lower end of a respective one of the stretchers to the runner. When the umbrella is closed, each of the ribs and the stretchers is collapsed in a tangential angle to set at one side of the central shaft and the runner and is not interfered with by the central shaft and the runner.