An electric power transmission device for wirelessly transmitting electric power in a highly conductive medium includes a power transmitting unit configured to wirelessly transmit electric power and a power receiving unit configured to intromit the wireless electric power transmitted from the power transmitting unit. The power transmitting unit and the power receiving unit include an electric power transmission coil and a containment member having a dielectric configured to cover the electric power transmission coil, and transmit the electric power by causing resonance at a frequency determined by impedance of the power transmitting unit, impedance of the power receiving unit, and impedance of the highly conductive medium.