A clutch device with a cam mechanism is provided, which is free from drag loss when the phase is switched from a state wherein the cam mechanism is operative to a state wherein it is inoperative. A cam groove 1c and a recess 1d are formed on the inner surface 1a of a piston, and a cam groove 3c and a projection 3d to be inserted into the recess 1d are formed on the inner surface 3a of a pressure plate 3. Between the cam grooves 1c and the 3c is interposed a cam roller 2 loosely retained by a roller cage 4. When the hydraulic pressure is applied to the back surface 1b of the piston 1 while a C.sub.3 clutch disk 5 rotates faster than the piston 1, the cam mechanism is operative to carry out the engagement operation. On the contrary, when the application of the hydraulic pressure is interrupted while the C.sub.3 clutch disk 5 rotates slower than the piston 1, the engagement is released. Meanwhile, the riding of the cam roller 2 over the cam elements is avoidable by the recess 1d and the projection 3d.