An antenna unit includes an antenna, a high-frequency amplifier, an attenuator circuit, an output cable, and a voltage detector circuit. An operating voltage is supplied from a receiver to the high-frequency amplifier through the output cable. The operating voltage changes in accordance with an automatic gain control (AGC) voltage. The voltage detector circuit detects a change in the operating voltage. In response to the voltage detector circuit output, one of the high-frequency amplifier and the attenuator circuit is selectively connected to a signal line between the antenna and the output cable.