An automatic transmission is provided with at least one friction element and a piston. The engagement of the friction element is proceeded in a manner that operating pressure is applied to an apply chamber to stroke the piston. The disengagement of the friction element is proceeded in a manner that operating pressure is applied to a cancel chamber to stroke the piston. The cancel chamber is disposed opposite to the apply chamber with respect to the piston and formed such that an outer diameter of a pressure receiving surface of said cancel chamber is generally the same as that of said apply chamber. Therefore, piston pushing force due to centrifugal force in the apply chamber is balanced with a piston reversely pushing force due to the centrifugal force in the cancel chamber. This prevents the friction element from being dragged during friction element disengaged condition.