A reflection mirror antenna device comprising: at least one primary radiator configured to operate in a first frequency band and in a second frequency band higher than the first frequency band, wherein the first frequency band is a transmission band and the second frequency band is a reception band; and a reflection mirror having a surface of a paraboloid of revolution configured to form a first beam in the first frequency band and a second beam in the second frequency band by reflecting radio waves in the first and second frequency bands, wherein a first region of the reflection mirror including a center point of the paraboloid of revolution is formed of a conductor, a second region, which is an outer peripheral side of the first region, of the reflection mirror is a region where a plurality of reflection elements, which are conductor patterns, is arranged on a dielectric body overlaid on a base plate conductor without forming a step on the mirror surface of the reflection mirror, the plurality of reflection elements arranged in the second region is configured to cause phase difference between a reflection phase of a radio wave on the first region and a reflection phase of a radio wave on the second region, and the phase difference between a reflection phase of a radio wave at the center point included in the first region and a reflection phase of a radio wave on the second region is in a range between 90 and 180 degrees, wherein an arrangement pitch of the plurality of reflection elements corresponds to a wavelength of a radio wave in the second frequency band, and wherein the at least one primary radiator is configured to to increase a power of the first beam radiated in the first frequency band in order to increase a gain of the first beam in the first frequency band, so that the gain of the first beam in the first frequency band is adjusted to coincide with a gain of the second beam in the second frequency band at ends of a coverage area. at least one primary radiator configured to operate in a first frequency band and in a second frequency band higher than the first frequency band, wherein the first frequency band is a transmission band and the second frequency band is a reception band; and a reflection mirror having a surface of a paraboloid of revolution configured to form a first beam in the first frequency band and a second beam in the second frequency band by reflecting radio waves in the first and second frequency bands, wherein a first region of the reflection mirror including a center point of the paraboloid of revolution is formed of a conductor, a second region, which is an outer peripheral side of the first region, of the reflection mirror is a region where a plurality of reflection elements, which are conductor patterns, is arranged on a dielectric body overlaid on a base plate conductor without forming a step on the mirror surface of the reflection mirror, the plurality of reflection elements arranged in the second region is configured to cause phase difference between a reflection phase of a radio wave on the first region and a reflection phase of a radio wave on the second region, and the phase difference between a reflection phase of a radio wave at the center point included in the first region and a reflection phase of a radio wave on the second region is in a range between 90 and 180 degrees, an arrangement pitch of the plurality of reflection elements corresponds to a wavelength of a radio wave in the second frequency band, and wherein the at least one primary radiator is configured to to increase a power of the first beam radiated in the first frequency band in order to increase a gain of the first beam in the first frequency band, so that the gain of the first beam in the first frequency band is adjusted to coincide with a gain of the second beam in the second frequency band at ends of a coverage area.