A binding device is provided which allows reduction in profile.A binding device 10 includes: a holding member 16 having a length which allows a plurality of openable-closable binding rings 12 and 14 to be disposed with a spacing therebetween; an operating member 18 secured inside the holding member 16; and an opening/closing member 40. The operating member 18 includes a pair of operating pieces 30 and 32 on which the binding rings 12 and 14 are secured. For housing the opening/closing member 40, an opening/closing member housing portion 36 is provided in a manner such as to be recessed from a reverse side, opposite to a surface on which the binding rings 12 and 14 are secured, towards the surface side. The opening/closing member 40 is secured to the opening/closing member housing portion 36 of the operating member 18 on the reverse side opposite to the surface on which the binding rings 12 and 14 are secured, and is formed to, when the binding rings 12 and 14 are opened, cause the binding rings 12 and 14 to be changed in an opening direction so that the operating pieces are moved in a longitudinal direction of the holding member 16 and are held in a direction approaching an inner surface of the holding member 16, within the holding member 16.