A brake device of a transmission is provided. The brake device includes a friction plate set including fixed friction plates and rotatable friction plates alternately disposed with each other, an engaging piston, and a clearance adjusting piston. A clearance adjusting hydraulic chamber is formed on a side of the clearance adjusting piston opposite to the friction plate set, and a concave portion is formed in a surface of the clearance adjusting piston on a friction plate set side. A base part of the engaging piston on a side opposite to the friction plate set is slidably fitted into the concave portion of the clearance adjusting piston and an engaging hydraulic chamber is formed by the base part and the concave part.