An electric power transmission device capable of efficiently performing contactless electric power transmission to an underwater vehicle in water is provided. The electric power transmission device, which is an electric power transmission device transmitting electric power to an electric power reception device including an electric power reception coil in water, includes one or more transmission coils including an electric power transmission coil which transmits electric power to an electric power reception coil via a magnetic field, an electric power transmission unit which transmits AC power to an electric power transmission coil, a capacitor which is connected to the transmission coil and forms a resonant circuit which resonates with the transmission coil, and a buoyant body connected to at least one of the transmission coils.