The present invention is to provide a binding processing apparatus that prevents easy unbinding regardless of thickness of a sheet bundle to be bound and that lessens influence of damage on sheets. In the present sheet binding processing apparatus, a pair of pressurizing faces for pressure-nipping a sheet bundle are formed to have tooth forms each having a convex shape and a concave shape, the tooth forms being engaged with each other along a predetermined tooth width, and the tooth forms are shaped so that an engagement depth between a tooth top and a tooth root is varied along a tooth width direction toward a sheet end edge.