A planar antenna comprising: a substrate formed of a dielectric; a distributed constant line formed on a first surface of the substrate, the distributed constant line including a first end to which power is supplied and a second end that is an open end or is grounded; and at least one first resonator (13-1 to 13-4; 23-1 to 23-4; 33-1 to 33-4) arranged on the first surface of the substrate and within a range in which the at least one first resonator (13-1 to 13-4; 23-1 to 23-4; 33-1 to 33-4) is allowed to be electromagnetically coupled to the distributed constant line in a vicinity of any of nodal points of a standing wave of a current that flows through the distributed constant line in response to a radio wave having a certain design wavelength radiated from the distributed constant line or received by the distributed constant line, wherein the at least one first resonator (13-1 to 13-4; 23-1 to 23-4; 33-1 to 33-4) includes a plurality of first resonators (13-1 to 13-4; 23-1 to 23-4; 33-1 to 33-4), the plurality of first resonators being each alternately arranged on the two sides of the conductor so as to sandwich the conductor, wherein each two first resonators which are adjacent on opposite sides of the conductor is arranged at an interval of half of the design wavelength between the two resonators in a range in which each of the two adjacent first resonators is allowed to be electromagnetically coupled to the distributed constant line, at respective two adjacent nodal points of a current that flows through the conductor, and wherein the design wavelength is the wavelength of the radio waves radiated from the antenna or received by the antenna. a substrate formed of a dielectric; a distributed constant line formed on a first surface of the substrate, the distributed constant line including a first end to which power is supplied and a second end that is an open end or is grounded; and at least one first resonator (13-1 to 13-4; 23-1 to 23-4; 33-1 to 33-4) arranged on the first surface of the substrate and within a range in which the at least one first resonator (13-1 to 13-4; 23-1 to 23-4; 33-1 to 33-4) is allowed to be electromagnetically coupled to the distributed constant line in a vicinity of any of nodal points of a standing wave of a current that flows through the distributed constant line in response to a radio wave having a certain design wavelength radiated from the distributed constant line or received by the distributed constant line, wherein the at least one first resonator (13-1 to 13-4; 23-1 to 23-4; 33-1 to 33-4) includes a plurality of first resonators (13-1 to 13-4; 23-1 to 23-4; 33-1 to 33-4), the plurality of first resonators being each alternately arranged on the two sides of the conductor so as to sandwich the conductor, wherein each two first resonators which are adjacent on opposite sides of the conductor is arranged at an interval of half of the design wavelength between the two resonators in a range in which each of the two adjacent first resonators is allowed to be electromagnetically coupled to the distributed constant line, at respective two adjacent nodal points of a current that flows through the conductor, and wherein the design wavelength is the wavelength of the radio waves radiated from the antenna or received by the antenna. wherein each two first resonators which are adjacent on opposite sides of the conductor is arranged at an interval of half of the design wavelength between the two resonators in a range in which each of the two adjacent first resonators is allowed to be electromagnetically coupled to the distributed constant line, at respective two adjacent nodal points of a current that flows through the conductor, and wherein the design wavelength is the wavelength of the radio waves radiated from the antenna or received by the antenna.