A clutch device configured for a transmission may include a first body and a second coaxially mounted to the first body; a friction element mounted between the first body and the second body; a friction piston configured to axially press the friction element by use of hydraulic pressure; a retractable mechanism including a dog clutch, which is configured to restrict rotation between the first body and the second body, and repeatedly engaging and disengaging the dog clutch by repeatedly applying pressure in axial direction thereof; and an operation piston configured to provide pressure to the retractable mechanism in an axial direction of the retractable mechanism.