In an antenna structure 1 in which a feed radiation electrode provided on a dielectric base member performs an antenna operation in a fundamental mode and an antenna operation in a higher-order mode with a resonant frequency higher than that in the fundamental mode, one end of the feed radiation electrode defines a feed end connected to a circuit for radio communication, and the other end of the feed radiation electrode defines an open end. The position of a capacitance-loading portion .alpha. is set in advance between the feed end and the open end of the feed radiation electrode. A capacitance-loading conductor is connected to one or both of the feed end and the capacitance-loading portion .alpha. of the feed radiation electrode. The capacitance-loading conductor forms a capacitance for adjusting a resonant frequency in the fundamental mode between the feed end and the capacitance-loading portion .alpha..