The objective of the present invention is to provide a connection cable that serves as an external antenna for a radio apparatus, and whose antenna performance is not deteriorated in the use state by an adverse affect attributable to the near proximity of a human body.According to the present invention, an antenna device, integrally formed with first connection means, which is to be connected to a radio apparatus, second connection means, which is to be connected to a peripheral device, and a connection cable and an antenna element, which are located in between and which transmit a plurality of signals from the radio apparatus to the peripheral device, includes: first relay means, located at a middle position in the connection cable, for relaying the plurality of signals; the antenna element, located between the second connection means and the first relay means; and a coaxial line, located between the first connection means and the first relay means for transmitting, to the radio apparatus, an antenna reception signal that has been received at the antenna element and has been extracted by the first relay means, wherein the antenna element serves as an external antenna for the radio apparatus.