A circularly polarized antenna comprising: an antenna substrate formed with a flat film conductor including a loop conductor and a slot conductor formed with a slot the flat film conductor being configured to transmit and receive a circularly polarized wave, and the slot conductor defining a radiation surface of the antenna substrate , wherein the loop conductor and the slot conductor are in a stacked arrangement, overlapping each other, and the center of the loop conductor is substantially in agreement with the center of the slot conductor when viewed from a direction orthogonal to the radiation surface; and a cavity configured: to be formed at an opposite side from the radiation surface of the antenna substrate , to at least partially overlap with the flat film conductor when seen in a depth direction of the cavity , and to have a size at least in one direction shorter than half of a wavelength of the circularly polarized wave, wherein the cavity includes a first cavity , the first cavity being configured to fully overlap with the loop conductor when seen in the depth direction of the cavity , the size of the first cavity in the one direction is the size of the first cavity in a direction orthogonal to the depth direction of the first cavity , and characterized in that the cavity includes a second cavity , the second cavity being separated from the first cavity and 25 configured to at least partially overlap with the slot conductor when seen in the depth direction of the cavity . an antenna substrate formed with a flat film conductor including a loop conductor and a slot conductor formed with a slot the flat film conductor being configured to transmit and receive a circularly polarized wave, and the slot conductor defining a radiation surface of the antenna substrate , wherein the loop conductor and the slot conductor are in a stacked arrangement, overlapping each other, and the center of the loop conductor is substantially in agreement with the center of the slot conductor when viewed from a direction orthogonal to the radiation surface; and a cavity configured: to be formed at an opposite side from the radiation surface of the antenna substrate , to at least partially overlap with the flat film conductor when seen in a depth direction of the cavity , and to have a size at least in one direction shorter than half of a wavelength of the circularly polarized wave, wherein the cavity includes a first cavity , the first cavity being configured to fully overlap with the loop conductor when seen in the depth direction of the cavity , the size of the first cavity in the one direction is the size of the first cavity in a direction orthogonal to the depth direction of the first cavity , and characterized in that the cavity includes a second cavity , the second cavity being separated from the first cavity and 25 configured to at least partially overlap with the slot conductor when seen in the depth direction of the cavity . to be formed at an opposite side from the radiation surface of the antenna substrate , to at least partially overlap with the flat film conductor when seen in a depth direction of the cavity , and to have a size at least in one direction shorter than half of a wavelength of the circularly polarized wave, wherein the cavity includes a first cavity , the first cavity being configured to fully overlap with the loop conductor when seen in the depth direction of the cavity , the size of the first cavity in the one direction is the size of the first cavity in a direction orthogonal to the depth direction of the first cavity , and characterized in that the cavity includes a second cavity , the second cavity being separated from the first cavity and 25 configured to at least partially overlap with the slot conductor when seen in the depth direction of the cavity . <EOS>