To provide a diversity system for efficiently switching a transmission / reception antenna and maintaining the quality of transmission / reception signals.

In reception, a selection switch A1 is switched so as to connect an antenna 2a to a reception circuit 3a and the antenna 2b to the reception circuit 3b and signals are received. At this point, a diversity selection circuit 19 compares "RSSI1" and "RSSI2" calculated by RSSI detection circuits 18a and 18b and switches the selection switch B11 so that the output signals of the reception circuit 3a is inputted to a demodulation circuit when "RSSI1" is larger, whereas the diversity selection circuit 19 switches the selection switch B11 so that the output signals of the reception circuit 3b is inputted to a demodulation circuit when "RSSI2" is larger. In transmission, the selection switch A1 is switched so that a transmission circuit 4 is connected to the antenna connected to the reception circuit which outputs the output signals to the demodulation circuit through the selection switch B11 at the reception immediately before.