Provided are a power transmitting device, a power receiving device, a power supply system, and a power supply method able to supply electric power by emitting electromagnetic waves. A power transmitting device comprises: a calculating unit for calculating the maximum value for the emitted output of electromagnetic waves meeting exposure standards on the basis of a response delay time measured by the communication link between the power transmitting device and a power receiving device; a power transmitting unit for transmitting power via a power supply link with the power receiving device at an output not exceeding the maximum value; an anomaly detecting unit for detecting an anomaly in the power supply link on the basis of communication with the power receiving device via the communication link; and an output control unit for controlling the output on the basis of the detection of an anomaly in the power supply link.