To reduce degradation in antenna electric performance by specifying the phase difference between a radio wave transmitted through a support structure and an electric wave passing through others.

The support structure 2 and a phase adjusting plate 5 in the passing range of a radio wave are made of dielectric such as aramid-fiber reinforced plastic, so a radio wave is transmitted. The plate thickness of the support structure 2 is 1 mm on one side and if the radio wave is delayed in phase by π/2 when transmitted through the 1 mm, it is considered that the radio wave after being transmitted through the support structure 2 is delayed in phase by π(=π/2×2) on both the sides. Here, the phase adjusting plate 5 of 2 mm in thickness is fitted and then the phase is further π delayed, so the radio wave transmitted through the support structure 2 and phase adjusting plate 5 has a phase difference 2π from the radio wave in an area where the support structure 2 is absent and the phase shift is eliminated. This phase adjusting plate 5 can have proper thickness and a proper shape and be made lightweight by selecting the dielectric such as aluminum, glass fiber, and aramid reinforced fiber in conformity to conditions.