An antenna coupling apparatus, an external-antenna connecting apparatus, and an onboard external-antenna connecting apparatus which inhibits unnecessary power emissions, reduces transmission loss, and offers high durability and high ease of operation. has an electromagnetic coupling element consisting of a conductor that is not in contact with a first antenna with respect to DC components and a ground conductor element consisting of a conductor that is not in contact with the first antenna with respect to DC components and in contact with a ground conductor with respect to DC components and reflecting the power transmitted or received by the first antenna toward the electromagnetic coupling element. An external-antenna connecting apparatus and an onboard external-antenna connecting apparatus have an electromagnetic coupling circuit in their bodies that electromagnetically couples the antenna of a portable radio terminal and an external antenna, with the external-antenna connecting apparatus and the onboard external-antenna connecting apparatus, being not in contact with the portable radio terminal with respect to DC components, when the portable radio terminal is secured in the bodies.