In a brake device of a transmission, rotation side friction plates are engaged with a cylindrical part of a rotation side holding member, and fixed side friction plates are engaged with a spline part that is provided on an outer peripheral surface of an inner cylindrical part of a fixed side holding member opposed to a cylindrical part of the rotation side holding member. The fixed side holding member includes an inflow oil passage that supplies lubricating oil from the outside to the inside of the radial direction thereof. An annular space communicating with an inflow oil passage is provided on the inside of the cylindrical part of the fixed side holding member. Oil discharge holes supplying lubricating oil, which was flowed into the annular space, to the rotation side and fixed side friction plates are provided on the cylindrical part of the fixed side holding member.