To enhance the degree of freedom from an aspect of the design of a binder for locking paper sheets with one or a plurality of closed rings, and to apply the binder to various sizes at low cost.

The binder 1, which is equipped with one or a plurality of closed rings 3 and binds paper sheets by the locking of them with the rings 3, holds the rings 3 and has a base 4 formed separately from the rings 3. The base 4 has a ring holding part 4a, in which a non-binding region 3a being a region on the side not binding paper sheets of the rings 3 is fitted to hold the rings, and a ring cover part 4b for covering the non-binding region 3a.