To provide a printing mechanism of a binding device capable of realizing the smooth conveyance and reliable printing of a binding tape and saving a space in a simple structure.

A printing head 502 is provided in an advancing/retractable manner with respect to a binding tape B in a printing part conveying passage 503, and a platen 6 is arranged at a position opposite to a printing unit across the printing part conveying passage 503. The platen 6 is turnable around a rotary shaft 603 by a solenoid 604. The width of the printing part conveying passage 503 is reduced during the printing mode. During the conveying mode of the binding tape B, the width of the printing part conveying passage 503 is increased at the inlet side and gradually reduced toward the outlet side.