A treatment-tool driving device that is configured to be attached, in an attachable/detachable manner, to a treatment tool such as an endoscope, said treatment tool being provided with at least one pair of tensile-force transmitting members for bending a bending portion provided at a distal end of an inserted portion of the treatment tool; said treatment-tool driving device being configured to drive the treatment tool by means of tensile forces imparted to the tensile-force transmitting members, the treatment-tool driving device comprising: two movable members (7; 29) that are configured to be individually attached, in an attachable/detachable manner, to ends of the individual tensile-force transmitting members; a driving source that is configured to generate motive power with which the treatment tool is configured to be driven; a motive-power transmitting mechanism (9; 28, 30, 31) that is configured to switch between coupling and decoupling between the driving source and the movable members (7; 29), and that is configured to transmit, in a coupled state, the motive power from the driving source so as to selectively impart tensile forces to the two movable members (7; 29) attached to the paired tensile-force transmitting members; and a biasing means that is configured to bias the two movable members (7; 29) in directions in which tensile forces are imparted to the tensile-force transmitting members in a state in which coupling by the motive-power transmitting mechanism (9; 28, 30, 31) is released; wherein the treatment-tool driving device further comprises: an intermediate-mechanism portion that is configured to be coupled with the treatment tool in an attachable/detachable manner, wherein the intermediate-mechanism portion is provided with: the movable members (7; 29); the motive-power transmitting mechanism (9; 28, 30, 31); and the biasing means, and wherein the motive-power transmitting mechanism (9; 28, 30, 31) is configured to switch between coupling and decoupling between the driving source and the movable members (7; 29) by attaching/detaching the driving source to/from the intermediate-mechanism portion. two movable members (7; 29) that are configured to be individually attached, in an attachable/detachable manner, to ends of the individual tensile-force transmitting members; a driving source that is configured to generate motive power with which the treatment tool is configured to be driven; a motive-power transmitting mechanism (9; 28, 30, 31) that is configured to switch between coupling and decoupling between the driving source and the movable members (7; 29), and that is configured to transmit, in a coupled state, the motive power from the driving source so as to selectively impart tensile forces to the two movable members (7; 29) attached to the paired tensile-force transmitting members; and a biasing means that is configured to bias the two movable members (7; 29) in directions in which tensile forces are imparted to the tensile-force transmitting members in a state in which coupling by the motive-power transmitting mechanism (9; 28, 30, 31) is released; wherein the treatment-tool driving device further comprises: an intermediate-mechanism portion that is configured to be coupled with the treatment tool in an attachable/detachable manner, wherein the intermediate-mechanism portion is provided with: the movable members (7; 29); the motive-power transmitting mechanism (9; 28, 30, 31); and the biasing means, and wherein the motive-power transmitting mechanism (9; 28, 30, 31) is configured to switch between coupling and decoupling between the driving source and the movable members (7; 29) by attaching/detaching the driving source to/from the intermediate-mechanism portion. an intermediate-mechanism portion that is configured to be coupled with the treatment tool in an attachable/detachable manner, wherein the intermediate-mechanism portion is provided with: the movable members (7; 29); the motive-power transmitting mechanism (9; 28, 30, 31); and the biasing means, and the movable members (7; 29); the motive-power transmitting mechanism (9; 28, 30, 31); and the biasing means, and wherein the motive-power transmitting mechanism (9; 28, 30, 31) is configured to switch between coupling and decoupling between the driving source and the movable members (7; 29) by attaching/detaching the driving source to/from the intermediate-mechanism portion.