A treatment-tool driving device being attached to a treatment tool provided with at least one pair of tensile-force transmitting members and driving the treatment tool by tensile forces imparted to the tensile-force transmitting members, wherein the device includes: movable members being individually attached to one ends of the individual tensile-force transmitting members; a driving source generating a motive power with which the treatment tool is driven; a motive-power transmitting mechanism switching between coupling and decoupling between the driving source and the movable members, and transmitting, in a coupled state, the motive power from the driving source so as to selectively impart tensile forces to the two movable members attached to the paired tensile-force transmitting members; and a biasing portion biasing the two movable members 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 is released.