A microwave heating device comprising: a heating chamber for accommodating a heating-target object; a microwave generating unit for generating microwaves; and a waveguide structure antenna having a ceiling surface and side wall surfaces (10a, 10b, 10c) defining a waveguide structure section, and a front opening, the waveguide structure antenna emitting the microwaves from the front opening into the heating chamber, wherein the waveguide structure antenna includes a coupling part joined to the ceiling surface to couple the microwaves into an internal space of the waveguide structure section, and the waveguide structure section includes at least one microwave extraction opening formed on the ceiling surface to emit circularly polarized waves from the microwave extraction opening into the heating chamber, a driving unit for rotating the waveguide structure antenna, wherein the coupling part includes a coupling shaft coupled to the driving unit to include a center of rotation of the waveguide structure antenna, characterized by a flange provided around the coupling shaft to configure the joining portion, the flange having a shorter length in the direction of a pipe axis of the waveguide structure section than a length in the direction orthogonal to the direction of the pipe axis, the pipe axis being a center line of the waveguide structure section, which passes through a center of cross section of the waveguide structure section and extends in microwave transmission direction, or a cross section of the coupling part at the joining portion has a shorter length in the direction of a pipe axis of the waveguide structure section than a length in the direction orthogonal to the direction of the pipe axis, the pipe axis being a center line of the waveguide structure section, which passes through a center of cross section of the waveguide structure section and extends in microwave transmission direction. a heating chamber for accommodating a heating-target object; a microwave generating unit for generating microwaves; and a waveguide structure antenna having a ceiling surface and side wall surfaces (10a, 10b, 10c) defining a waveguide structure section, and a front opening, the waveguide structure antenna emitting the microwaves from the front opening into the heating chamber, wherein the waveguide structure antenna includes a coupling part joined to the ceiling surface to couple the microwaves into an internal space of the waveguide structure section, and the waveguide structure section includes at least one microwave extraction opening formed on the ceiling surface to emit circularly polarized waves from the microwave extraction opening into the heating chamber, a driving unit for rotating the waveguide structure antenna, wherein the coupling part includes a coupling shaft coupled to the driving unit to include a center of rotation of the waveguide structure antenna, characterized by a flange provided around the coupling shaft to configure the joining portion, the flange having a shorter length in the direction of a pipe axis of the waveguide structure section than a length in the direction orthogonal to the direction of the pipe axis, the pipe axis being a center line of the waveguide structure section, which passes through a center of cross section of the waveguide structure section and extends in microwave transmission direction, or a cross section of the coupling part at the joining portion has a shorter length in the direction of a pipe axis of the waveguide structure section than a length in the direction orthogonal to the direction of the pipe axis, the pipe axis being a center line of the waveguide structure section, which passes through a center of cross section of the waveguide structure section and extends in microwave transmission direction.