An automatic transmission includes: a brake device including a friction plate set configured with fixation friction plates and rotation friction plates alternately arranged, a piston, and an oil pressure chamber supplied with oil pressure that moves the piston toward the friction plate set side; a first rotating member; and second and third rotating members arranged at both respective axial sides of the first rotating member and each having an outer diameter smaller than that of the first rotating member. The friction plate set and the oil pressure chamber are arranged at both respective axial sides of the first rotating member and also arranged at an outer periphery side of the second rotating member and at the outer periphery side of the third rotating member, respectively. The piston extends from the oil pressure chamber toward the friction plate set side through the outer periphery side of the first rotating member.