To provide a simply structured inferior binding detecting device for surely detecting inferior binding.

A tab 18 is movably supported by a press member 11 able to go up and down for pressing the top face of sheets A stacked before binding the sheets A. The sheets A and the tab 18 are bound in a condition where the tip end of the tab 18 is placed at the binding position of the sheets A. After binding, the tab 18 is raised together with the press member 11, which causes a detection sensor PH to detect whether the sheets A are raised or not. When the sensor PH detects that the sheets A are not raised, the sensor PH outputs a signal of inferior binding.