A microwave heating device comprising: a heating chamber which is adapted to house an object to be heated and to direct microwaves to the object to be heated for performing high-frequency heating; a microwave feeding chamber formed to protrude upwardly from a ceiling wall surface of the heating chamber; a microwave generating portion adapted to create microwaves for performing high-frequency heating on the object to be heated, within the heating chamber; a waveguide adapted to couple the feeding chamber to the microwave generating portion for propagating microwaves; and a feeding portion including a vertical shaft element provided in a vertical direction by penetrating through a coupling hole formed in portions at which the feeding chamber and the waveguide are bonded to each other, and a flat-plate element having a radiation surface for radiating microwaves within the heating chamber, the flat-plate element being bonded to the vertical shaft element, wherein at least a partial radiation surface, out of the microwave radiation surface of the flat-plate element, is placed to be inclined at a predetermined angle θ with respect to a horizontal direction, characterized in that the radiation surface at the predetermined angle θ is made to have an area which occupies 1/2 or more of the entire radiation surface of the flat-plate element, assuming that Ly is a total length of the radiation surface inclined at the predetermined angle θ with respect to a horizontal plane, in the direction of the inclination, out of the entire radiation surface of the flat-plate element, and H is a height from the object to be heated within the heating chamber to a position in the radiation surface of the flat-plate element which is coincident with the position where the flat-plate element is bonded to the vertical shaft element, the inclination angle θrad of the inclined radiation surface is set to be an angle which is larger than Ly/2/H but is smaller than Ly/H. a heating chamber which is adapted to house an object to be heated and to direct microwaves to the object to be heated for performing high-frequency heating; a microwave feeding chamber formed to protrude upwardly from a ceiling wall surface of the heating chamber; a microwave generating portion adapted to create microwaves for performing high-frequency heating on the object to be heated, within the heating chamber; a waveguide adapted to couple the feeding chamber to the microwave generating portion for propagating microwaves; and a feeding portion including a vertical shaft element provided in a vertical direction by penetrating through a coupling hole formed in portions at which the feeding chamber and the waveguide are bonded to each other, and a flat-plate element having a radiation surface for radiating microwaves within the heating chamber, the flat-plate element being bonded to the vertical shaft element, wherein at least a partial radiation surface, out of the microwave radiation surface of the flat-plate element, is placed to be inclined at a predetermined angle θ with respect to a horizontal direction, characterized in that the radiation surface at the predetermined angle θ is made to have an area which occupies 1/2 or more of the entire radiation surface of the flat-plate element, assuming that Ly is a total length of the radiation surface inclined at the predetermined angle θ with respect to a horizontal plane, in the direction of the inclination, out of the entire radiation surface of the flat-plate element, and H is a height from the object to be heated within the heating chamber to a position in the radiation surface of the flat-plate element which is coincident with the position where the flat-plate element is bonded to the vertical shaft element, the inclination angle θrad of the inclined radiation surface is set to be an angle which is larger than Ly/2/H but is smaller than Ly/H.