A cylinder device includes a regulation mechanism that regulates separation distance between a front piston and a rear piston. A spring seat member of the regulation mechanism includes a spring reception part that projects toward a retainer side to receive a rear end of a spring, a cup seal holding part that projects on an opposite side from the retainer to hold a cup seal arranged on the rear piston, and a projection part that projects on the opposite side from the retainer. The projection part becomes abuttable on the retainer by a stroke of the rear piston if the spring reception part and the cup seal holding part are installed reversely-oriented on the rear piston.