There is provided a reduction gear device applied to a vehicle having a motor unit with a motor shaft for outputting a driving force for driving a driving wheel. The reduction gear device includes an input shaft operatively connected to said motor shaft; a reduction gear unit operatively connected to said input shaft; a brake disk supported by the input shaft in a relatively non-rotatable and axially movable manner; an operating member for pushing and moving the brake disk toward a braking surface; a housing for accommodating the reduction gear unit, the brake disk and the operating member; and a brake operating shaft which is supported rotatably about an axis by the housing along a radial direction of the brake disk and has an engagement part engaging with the operating member. In the reduction gear device, the engagement part of the brake operating shaft is in a non-circular shape in a cross-sectional view, in order that the operating member pushes and moves the brake disk despite the brake operating shaft being rotated in any direction about the axis.