A power transmitting device transmits power to a power receiving device having a power receiving coil, in water. The power transmitting device includes one or more annular transmitting coils including a power transmitting coil configured to transmit the power to the power receiving coil via a magnetic field, a support member located in an inner space of the transmitting coil, which is formed by the annular transmitting coil, the support member including an outer periphery along an inner periphery of the annular transmitting coil, and supporting the transmitting coil from the inner space, a power transmitting unit configured to supply A/C power to the power transmitting coil, and a capacitor connected to the transmitting coil, and forming a resonance circuit resonating together with the transmitting coil.