Abstract: A mobile terminal device includes a circuit board accommodated in a casing and an antenna disposed on the outside of the casing electrically connected to each other via a opening formed in the casing, a first conductive member fixed to an inner surface of the casing while being connected to the circuit board, a second conductive member detachably attached to an outer surface of the casing while being connected to the antenna, and a conductive fastening member fitted into the opening from outside the casing and fastening the first conductive member and the second conductive member together.