This invention relates to a single-way broad band mm wave plane slot antenna, in which, a coplanar waveguide feed line and a rectangle slot radiation unit are set at a same plane on a side of a microwave base, a beam guiding slot is set on the other side of the base, the rectangle cut-off slot is set under the base closing to the rectangle slot radiation unit on a supporting structure, the beam guiding slot, the rectangle radiation slot unit and three-layer or over rectangle cut-off slots are concentric forming similar rectangles getting smaller from the top, this invented coplanar waveguide feed line and impedance can be adjusted easily and the feed part and the radiation unit are coplanar, the rectangle slot radiation unit can reach the requirement of broadband and single-way radiation can be realized by three-layer or more coplanar rectangle cut-off slot reflection structure steadily reduced at the back and a positive wavebeam leading structure.