A lock-up device 7 includes a front cover 2, an impeller 3, a turbine 4, and a piston 22 positioned to divide a space between the front cover 2 and the turbine 4, and is engageable with the front cover 2. The piston 22 is formed with a plurality of projections 41 extending in the radial direction on a surface 42. The projection 41 has a straight front edge 51 and a straight rear edge 52, and an arced radially outer edge 53 and radially inner edge 54. The rear edge 52 in the rotation direction overlaps a straight line Ls connecting a radially inner end 52a with the rotation center O, or the rear edge 52 in the rotation direction is inclined to the straight line Ls such that a radially outer end 52b is positioned rearward of the straight line Ls in the rotation direction.