A bookbinding device includes a plurality of annular binders corresponding to binding holes. A part of an annular binder is inserted into a binding hole to be engaged, and by virtue of the insertion of the annular binder into the corresponding binding hole, the sheet bundle is bound to be provided as a booklet. Respective ends of binding members engage with each other, whereby the binding members are unified. The binding members are disposed so as to be aligned and constitute respectively binding member units which can be separated one by one. The pairs of the binding members are located in sequence so as to correspond to the binding holes to be pushed and moved and, thus, to be inserted through the binding holes. The respective ends of the binding members then engage with each other, whereby the binding members are unified.