A source device includes a first control unit to perform a negotiation with the sink device based on power standard and perform control of supplying power to a sink device based on first power information determined by the negotiation. The source device includes a power detection unit to detect power required by the sink device and includes a second control unit to receive the first power information including voltage and current values of power determined by the negotiation and of second power information including voltage and current values of power detected. The second control unit generates third power information, including voltage and current values based on the inputs of the first power information and the second power information and on the predetermined power standard. The second control unit instructs the first control unit to perform negotiation again on the basis of the third power information generated.