To obtain an antenna device which can improve performance while suppressing an increase in cost.

The antenna device comprises: an antenna substrate 1 on which a transmission antenna 2 and a reception antenna 3 are formed; and a radome 4 for covering the antenna substrate 1. On the antenna substrate 1, the transmission antenna 2 and the reception antenna 3 are formed so that a polarization plane of the transmission antenna 2 and a polarization plane of the reception antenna 3 are orthogonal to each other. The radome 4 comprises polarizers 5 and 6 for rotating a polarization plane of a first radio wave and a polarization plane of a second radio wave so that the polarization plane of the first radio wave which the transmission antenna 2 has transmitted and the polarization plane of the second radio wave which the reception antenna 3 receives are parallel with each other on the outside of the radome 4, and they are orthogonal to each other on the inside of the radome 4.