A microwave plasma-treating apparatus which generates plasma in a treating chamber by feeding microwave electric power thereto, comprising: a rectangular waveguide for feeding microwave electric power; a plurality of microwave coupling holes (71a, 71b, 71c, 71d, 71e, 81a, 81b, 81c, 81d) capable of transmitting microwaves formed in lower surfaces of said waveguide in the axial direction thereof arranged to maintain a distance of a half wavelength of standing waves of the microwaves therebetween, each of said microwave coupling holes including an annular shape having an island-shaped shield at a central portion, each of said microwave coupling holes comprising two or more holes, wherein said annular shape of each of said microwave coupling holes has inner diameters and outer diameters, and wherein said inner diameters and outer diameters have sizes which are controlled so as to maintain uniformity of plasma density; a dielectric window (72, 82) capable of transmitting microwaves arranged for said microwave coupling holes in an axial direction of said waveguide under said microwave coupling holes said dielectric window being arranged to provide a gap (77, 87) between said dielectric window and said lower surfaces of said waveguide; and a cooling member for cooling said dielectric window by providing cooling gas into said gap, wherein said waveguide includes a cooling gas feed hole provided at one end portion of the upper surface of the waveguide, and wherein said waveguide is arranged so that when said cooling gas is fed into said waveguide, it passes through said cooling gas feed hole, partly flows in said waveguide, partly is introduced into the gap through the coupling holes and flows therein in the axial direction of the waveguide, and is exhausted to the exterior passing through a cooling gas exhaust hole. a rectangular waveguide for feeding microwave electric power; a plurality of microwave coupling holes (71a, 71b, 71c, 71d, 71e, 81a, 81b, 81c, 81d) capable of transmitting microwaves formed in lower surfaces of said waveguide in the axial direction thereof arranged to maintain a distance of a half wavelength of standing waves of the microwaves therebetween, each of said microwave coupling holes including an annular shape having an island-shaped shield at a central portion, each of said microwave coupling holes comprising two or more holes, wherein said annular shape of each of said microwave coupling holes has inner diameters and outer diameters, and wherein said inner diameters and outer diameters have sizes which are controlled so as to maintain uniformity of plasma density; a dielectric window (72, 82) capable of transmitting microwaves arranged for said microwave coupling holes in an axial direction of said waveguide under said microwave coupling holes said dielectric window being arranged to provide a gap (77, 87) between said dielectric window and said lower surfaces of said waveguide; and a cooling member for cooling said dielectric window by providing cooling gas into said gap, wherein said waveguide includes a cooling gas feed hole provided at one end portion of the upper surface of the waveguide, and wherein said waveguide is arranged so that when said cooling gas is fed into said waveguide, it passes through said cooling gas feed hole, partly flows in said waveguide, partly is introduced into the gap through the coupling holes and flows therein in the axial direction of the waveguide, and is exhausted to the exterior passing through a cooling gas exhaust hole. wherein said waveguide includes a cooling gas feed hole provided at one end portion of the upper surface of the waveguide, and wherein said waveguide is arranged so that when said cooling gas is fed into said waveguide, it passes through said cooling gas feed hole, partly flows in said waveguide, partly is introduced into the gap through the coupling holes and flows therein in the axial direction of the waveguide, and is exhausted to the exterior passing through a cooling gas exhaust hole.