A circularly polarized antenna (10A, 10B, 10C) comprising: an antenna substrate formed with a flat film conductor (22,23) 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 (32,35) 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 (22,23) when seen in a depth direction of the cavity (32,35), and to have a size at least in one direction shorter than half of a wavelength of the circularly polarized wave, wherein the cavity (32,35) 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 (32,35), 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 (32,35) 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 (22,23) 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 (32,35) 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 (22,23) when seen in a depth direction of the cavity (32,35), and to have a size at least in one direction shorter than half of a wavelength of the circularly polarized wave, wherein the cavity (32,35) 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 (32,35), 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 (32,35) 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 (22,23) when seen in a depth direction of the cavity (32,35), and to have a size at least in one direction shorter than half of a wavelength of the circularly polarized wave, wherein the cavity (32,35) 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 (32,35), 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 (32,35) 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.