A lock-up device for a torque converter transmits a torque from a front cover to a transmission-side member through a turbine. The lock-up device includes a clutch portion, a piston, a support member, and a synchronization mechanism. The clutch portion is disposed between the front cover and the turbine. The piston is movable in an axial direction. The piston turns the clutch portion into a torque transmission state. The support member supports the piston such that the piston is movable. The support member defines an oil chamber together with the piston. The oil chamber is supplied a hydraulic oil for activating the piston. The synchronization mechanism causes the piston to rotate in synchronization with the support member.