In an automatic transmission including a deceleration planetary gear and a hydraulic clutch, the hydraulic clutch includes a clutch drum, a clutch hub, first and second friction engagement plates, a piston, and an engagement oil chamber defining member. The clutch hub is formed by a ring gear and a support member that rotates together with the ring gear. The piston is movably supported by the clutch hub. The piston and the support member define a cancel oil chamber that cancels a centrifugal oil pressure generated in an engagement oil chamber.