The antenna includes a first ground plate, a first dielectric substrate formed on the first ground plate, a transmission line made of a conductive material formed on the first dielectric substrate, and a plurality of antenna elements electromagnetically coupled to the transmission line. The transmission line is constituted of at least one first line serving as a resonator having a resonator length equal to (2n−1)/2 times (n being a positive integer) a guide wavelength of the transmission line and a plurality of second lines each having an electrical length longer than half the guide wavelength, the first and second lines being disposed alternately at predetermined intervals. Each of the antenna elements is electromagnetically coupled to a corresponding one of the second lines.