A lock-up device for a torque converter, comprising: an input side clutch supporter fixed to a front cover for transmitting power from a drive side; a lock-up clutch having a plurality of clutch plates supported by the input side clutch supporter; a lock-up piston to pressurize the clutch plates to make the clutch connected; a piston guide supporting an end portion of the lock-up piston; a spring pack fixed to the lock-up piston and installed between the lock-up piston and the input side clutch supporter, the spring pack including: a first retainer plate installed on the side of the lock-up piston and fixed with the lock-up piston, a second retainer plate installed to the input side clutch supporter side, a plurality of first spring supporters installed on a surface of the first retainer plate facing the second retainer plate, a plurality of second spring supporters installed on a surface of the second retainer plate facing the first retainer plate, and a plurality of coil springs supported by the first spring supporters and the second spring supporters, the coil springs pressing the lock-up piston in the direction of releasing the clutch; and a mutual rotation prevention mechanism that prevents mutual rotation between the lock-up piston and the input side clutch supporter, wherein the mutual rotation prevention mechanism includes: an outer gear installed on the exterior circumference of the first retainer plate of the spring pack; and an inner gear installed in the inner circumference of the input side clutch supporter and engaged with the outer gear installed in the exterior circumference of the first retainer plate. an input side clutch supporter fixed to a front cover for transmitting power from a drive side; a lock-up clutch having a plurality of clutch plates supported by the input side clutch supporter; a lock-up piston to pressurize the clutch plates to make the clutch connected; a piston guide supporting an end portion of the lock-up piston; a spring pack fixed to the lock-up piston and installed between the lock-up piston and the input side clutch supporter, the spring pack including: a first retainer plate installed on the side of the lock-up piston and fixed with the lock-up piston, a second retainer plate installed to the input side clutch supporter side, a plurality of first spring supporters installed on a surface of the first retainer plate facing the second retainer plate, a plurality of second spring supporters installed on a surface of the second retainer plate facing the first retainer plate, and a plurality of coil springs supported by the first spring supporters and the second spring supporters, the coil springs pressing the lock-up piston in the direction of releasing the clutch; and a first retainer plate installed on the side of the lock-up piston and fixed with the lock-up piston, a second retainer plate installed to the input side clutch supporter side, a plurality of first spring supporters installed on a surface of the first retainer plate facing the second retainer plate, a plurality of second spring supporters installed on a surface of the second retainer plate facing the first retainer plate, and a plurality of coil springs supported by the first spring supporters and the second spring supporters, the coil springs pressing the lock-up piston in the direction of releasing the clutch; and a mutual rotation prevention mechanism that prevents mutual rotation between the lock-up piston and the input side clutch supporter, wherein the mutual rotation prevention mechanism includes: an outer gear installed on the exterior circumference of the first retainer plate of the spring pack; and an inner gear installed in the inner circumference of the input side clutch supporter and engaged with the outer gear installed in the exterior circumference of the first retainer plate. an outer gear installed on the exterior circumference of the first retainer plate of the spring pack; and an inner gear installed in the inner circumference of the input side clutch supporter and engaged with the outer gear installed in the exterior circumference of the first retainer plate.