To provide an inexpensive, long-life, and highly reliable antenna device converting a signal received by an antenna into an optical signal and transmitting it.

An output signal from a wave detecting means 3 detecting a wave of a received signal of an antenna 1 is converted into an optical signal by a semiconductor light emitting element 4. A wave detection signal is low in a frequency in comparison with the received signal of the antenna 1, so that an inexpensive long-life semiconductor light emitting element can be used. When bias current flows through the semiconductor light emitting element 4 from a direct current power source 5, alternating current component resistance is lowered, so that a minute current can be converted into an optical signal in the semiconductor light emitting element 4. As the optical signal passes through an optical fiber, a transmission distance can be prolonged.