To provide an antenna system capable of obtaining a desired signal level.

First and second antennas 2a, 2b belonging to a first antenna group 2 are arranged to transmit / receive signals with a high correlation and third and fourth antennas 8a, 8b belonging to a second antenna group 8 are similarly arranged. The first and second antenna groups 2, 8 are arranged to mutually obtain signals with a low correlation. Levels of radio waves received respectively by the first antenna 2a and the third antenna 8a are compared with each other and when the level of the radio wave received by the first antenna 2a is greater, the antenna group of the first antenna group 2 is used, and when the level of the radio wave received by the third antenna 8a is greater, the antenna group of the second antenna group 8 is used.