A microwave heater comprising: a heating chamber adapted to house an object to be heated; a placement portion which forms a bottom surface of the heating chamber and is adapted to accommodate and place, thereon, the object to be heated within the heating chamber; a microwave generating portion adapted to generate a microwave; and a waveguide adapted to propagate the microwave from the microwave generating portion; characterized in that: the microwave heater further comprises a plurality of microwave radiating portions which are provided in a surface of the waveguide which is faced to the heating chamber and are adapted to radiate circularly-polarized waves within the heating chamber, the plurality of the microwave radiating portions are placed, in such a way as to be arranged in a direction orthogonal to a direction of an electric field and to a direction of propagation in the waveguide, and each of the microwave radiating portions is constituted by at least two or more slits, and each of the microwave radiating portions be formed at positions deviated from the center of the waveguide in the direction orthogonal to the direction of propagation and to the direction of the electric field, wherein the slits are inclined with respect to the center axis of the waveguide in the direction of propagation. a heating chamber adapted to house an object to be heated; a placement portion which forms a bottom surface of the heating chamber and is adapted to accommodate and place, thereon, the object to be heated within the heating chamber; a microwave generating portion adapted to generate a microwave; and a waveguide adapted to propagate the microwave from the microwave generating portion; characterized in that: the microwave heater further comprises a plurality of microwave radiating portions which are provided in a surface of the waveguide which is faced to the heating chamber and are adapted to radiate circularly-polarized waves within the heating chamber, the plurality of the microwave radiating portions are placed, in such a way as to be arranged in a direction orthogonal to a direction of an electric field and to a direction of propagation in the waveguide, and each of the microwave radiating portions is constituted by at least two or more slits, and each of the microwave radiating portions be formed at positions deviated from the center of the waveguide in the direction orthogonal to the direction of propagation and to the direction of the electric field, the microwave heater further comprises a plurality of microwave radiating portions which are provided in a surface of the waveguide which is faced to the heating chamber and are adapted to radiate circularly-polarized waves within the heating chamber, the plurality of the microwave radiating portions are placed, in such a way as to be arranged in a direction orthogonal to a direction of an electric field and to a direction of propagation in the waveguide, and each of the microwave radiating portions is constituted by at least two or more slits, and each of the microwave radiating portions be formed at positions deviated from the center of the waveguide in the direction orthogonal to the direction of propagation and to the direction of the electric field, wherein the slits are inclined with respect to the center axis of the waveguide in the direction of propagation.