A microwave heating device comprising: a heating chamber for housing an object to be heated and for radiating a microwave toward the object to be heated for performing high-frequency heating on the object to be heated; a microwave generating part adapted to generate a microwave for performing high-frequency heating on the object to be heated within the heating chamber, and to output the generated microwave from an output part; a waveguide having a horizontal propagation path and a vertical propagation path orthogonal to each other such that the microwave generating part is horizontally coupled to the vertical propagation path, for propagating a microwave from the microwave generating part through the horizontal propagation path; a feeding part which is coupled to the horizontal propagation path and includes an antenna part for radiating, within the heating chamber, the microwave propagated through the waveguide; and an antenna room which is provided in a ceiling surface of the heating chamber, further is adapted to reflect the microwave radiated in a horizontal direction from the antenna part, and is opened at its lower end portion such that the microwave from the antenna part is radiated within the heating chamber; characterized in that: the feeding part is provided in a feeding port formed in the horizontal propagation path in the waveguide so that the feeding part is coupled to the waveguide, a horizontal propagation distance is defined as a horizontal distance from an inner folding position of the orthogonally-folded waveguide to the center of the feeding port in the horizontal propagation path in the waveguide, along a direction in which the horizontal propagation path extends, and a vertical propagation distance is defined as a vertical distance from the inner folding portion of the orthogonally-folded waveguide to the center of the output part of the microwave generating part wherein the waveguide is structured such that the horizontal propagation distance is longer than 1/2 the wavelength of the microwave which propagates through the waveguide and that the vertical propagation distance is smaller than 1/4 the wavelength of the microwave which propagates through the waveguide. a heating chamber for housing an object to be heated and for radiating a microwave toward the object to be heated for performing high-frequency heating on the object to be heated; a microwave generating part adapted to generate a microwave for performing high-frequency heating on the object to be heated within the heating chamber, and to output the generated microwave from an output part; a waveguide having a horizontal propagation path and a vertical propagation path orthogonal to each other such that the microwave generating part is horizontally coupled to the vertical propagation path, for propagating a microwave from the microwave generating part through the horizontal propagation path; a feeding part which is coupled to the horizontal propagation path and includes an antenna part for radiating, within the heating chamber, the microwave propagated through the waveguide; and an antenna room which is provided in a ceiling surface of the heating chamber, further is adapted to reflect the microwave radiated in a horizontal direction from the antenna part, and is opened at its lower end portion such that the microwave from the antenna part is radiated within the heating chamber; characterized in that: the feeding part is provided in a feeding port formed in the horizontal propagation path in the waveguide so that the feeding part is coupled to the waveguide, a horizontal propagation distance is defined as a horizontal distance from an inner folding position of the orthogonally-folded waveguide to the center of the feeding port in the horizontal propagation path in the waveguide, along a direction in which the horizontal propagation path extends, and a vertical propagation distance is defined as a vertical distance from the inner folding portion of the orthogonally-folded waveguide to the center of the output part of the microwave generating part wherein the waveguide is structured such that the horizontal propagation distance is longer than 1/2 the wavelength of the microwave which propagates through the waveguide and that the vertical propagation distance is smaller than 1/4 the wavelength of the microwave which propagates through the waveguide. the feeding part is provided in a feeding port formed in the horizontal propagation path in the waveguide so that the feeding part is coupled to the waveguide, a horizontal propagation distance is defined as a horizontal distance from an inner folding position of the orthogonally-folded waveguide to the center of the feeding port in the horizontal propagation path in the waveguide, along a direction in which the horizontal propagation path extends, and a vertical propagation distance is defined as a vertical distance from the inner folding portion of the orthogonally-folded waveguide to the center of the output part of the microwave generating part wherein the waveguide is structured such that the horizontal propagation distance is longer than 1/2 the wavelength of the microwave which propagates through the waveguide and that the vertical propagation distance is smaller than 1/4 the wavelength of the microwave which propagates through the waveguide.