PURPOSE: To see that the sheet materials accumulated on a bin are not turned over or broken by a stapler shifting for binding the same materials in a bundle.

CONSTITUTION: A largish cut 62 is provided in the section, where an electromotive stapler 61 performing the binding work is positioned, of a bin B,-and this cut 62 section is covered with a thin transparent synthetic resin sheet material 63. Moreover, a rectangular cut 64 for escaping a binding needle is provided in the position, where the binding needle is driven, of the synthetic resin sheet material 63.