A detection device (20, 20D) comprising: an antenna that is configured to transmit and receive an electric wave including a signal; a signal output unit that includes a first circuit terminal and a second circuit terminal which are connected to the antenna, that is configured to operate by using a power supply signal included in the signal that is received by the antenna as power, and that is configured to output a detection signal to the antenna; and a gain reduction unit (22, 24) that is configured to perform switching between a gain reduction state in which a gain of the antenna is reduced and a gain non-reduction state in which the gain of the antenna is not reduced in correspondence with a state of a detection target, wherein the gain reduction unit (22, 24) includes a first operation point (22a, 22b) configured for reducing the gain of the antenna within a range on the antenna from the first circuit terminal to a position that is spaced away from the first circuit terminal by λ/8 along a first extension direction of the antenna, and a second operation point (22d, 22e) configured for reducing the gain of the antenna within a range on the antenna from the second circuit terminal to a position that is spaced away from the second circuit terminal by λ/8 along a second extension direction of the antenna, wherein λ is a set wavelength of the electric wave, characterized in that the gain reduction unit includes a first capacitor circuit and a second capacitor circuit, the first capacitor circuit includes a first flat plate and a second flat plate as the first operation point (24a, 24b), the second capacitor circuit includes a third flat plate and a fourth flat plate as the second operation point (24c, 24d) wherein the first flat plate and the second flat plate are provided in a movable manner, and wherein the third flat plate and a fourth flat plate are provided in a movable manner, and wherein the gain reduction unit is configured to perform switching between the gain reduction state and the gain non-reduction state by causing both the first flat plate and the second flat plate, as well as the third flat plate and the fourth flat plate, to approach each other; and wherein the gain reduction unit is configured to perform switching between the non-gain reduction state and the gain reduction state by causing both the first flat plate and the second flat plate, as well as the third flat plate and the fourth flat plate, to be spaced away from each other. an antenna that is configured to transmit and receive an electric wave including a signal; a signal output unit that includes a first circuit terminal and a second circuit terminal which are connected to the antenna, that is configured to operate by using a power supply signal included in the signal that is received by the antenna as power, and that is configured to output a detection signal to the antenna; and a gain reduction unit (22, 24) that is configured to perform switching between a gain reduction state in which a gain of the antenna is reduced and a gain non-reduction state in which the gain of the antenna is not reduced in correspondence with a state of a detection target, wherein the gain reduction unit (22, 24) includes a first operation point (22a, 22b) configured for reducing the gain of the antenna within a range on the antenna from the first circuit terminal to a position that is spaced away from the first circuit terminal by λ/8 along a first extension direction of the antenna, and a second operation point (22d, 22e) configured for reducing the gain of the antenna within a range on the antenna from the second circuit terminal to a position that is spaced away from the second circuit terminal by λ/8 along a second extension direction of the antenna, wherein λ is a set wavelength of the electric wave, characterized in that the gain reduction unit includes a first capacitor circuit and a second capacitor circuit, the first capacitor circuit includes a first flat plate and a second flat plate as the first operation point (24a, 24b), the second capacitor circuit includes a third flat plate and a fourth flat plate as the second operation point (24c, 24d) wherein the first flat plate and the second flat plate are provided in a movable manner, and wherein the third flat plate and a fourth flat plate are provided in a movable manner, and wherein the gain reduction unit is configured to perform switching between the gain reduction state and the gain non-reduction state by causing both the first flat plate and the second flat plate, as well as the third flat plate and the fourth flat plate, to approach each other; and wherein the gain reduction unit is configured to perform switching between the non-gain reduction state and the gain reduction state by causing both the first flat plate and the second flat plate, as well as the third flat plate and the fourth flat plate, to be spaced away from each other.