To drive a staple needle into a sheet bundle with a light load, without requiring a wide storage space.

This staple needle 10 binds the sheet bundle by being separated by a stapler from a state of being separably connected in a plurality. The staple needle 101 has a pressing-down part 101a pressing down the sheet bundle, and a pair of penetrating parts 101b bendably arranged by extending from the pressing-down part 101a and penetrating through the sheet bundle. The pair of penetrating parts 101b are formed in such a divergent shape that an interval thereof increases toward the tip of the penetrating parts 101b from the pressing-down part 101a before use.