An antenna apparatus 1 includes a high-frequency output portion 2 for outputting a high-frequency signal, an antenna 5 including a first excitation unit 3 and a second excitation unit 4, the first excitation unit 3 emitting a first linearly polarized wave according to the high-frequency signal output from the high-frequency output portion 2, the second excitation unit 4 emitting a second linearly polarized wave that is orthogonal to the first linearly polarized wave at the same time with the first linearly polarized wave according to the high-frequency signal output from the high-frequency output portion, and a phase adjustment portion 6 for adjusting a phase of at least one of the high-frequency signal to be input to the first excitation unit 3 and the high-frequency signal to be input to the second excitation unit 4 in a range of change from 0 to 270 or more degrees.