Provided is a binder in which a binder is provided with a binding member having a pressing portion pressing a material to be bound, an operation member operated so as to pivotally rotate to an open state and a hold state, and a base member the binder is further provided with an engagement body, a projection portion caused to engage with the engagement body, and a supporting portion provided with the engagement body, the supporting portion is configured so as to bend toward the projection portion side and bend in a direction of separating from the projection portion and the locked state is formed by engagement between the engagement body and the projection portion.