According to an embodiment, a power transmitting device includes a power supply unit, a power transmitting coil, a first switch, a piezoelectric element, and a first switch controller. The power supply unit is configured to generate a transmission power by using power supplied to a power input terminal. Once the first switch is turned on, the first switch electrically connects an external power supply and the power input terminal until the first switch is turned off next time. The piezoelectric element is disposed at a position at which a weight of a placed power receiving device can be detected. The piezoelectric element is configured to generate a detection power according to the weight of the placed power receiving device. The first switch controller is configured to generate a starting power by the detection power and switches the first switch to on by using the starting power.