A power supply method of a sink device using a High Definition Multimedia Interface, HDMI, comprising: connecting to a source device through the HDMI; if the sink device determines (S8010) that a battery state of the source device is below a threshold by detecting that a +5 V signal is not received from the source device, supplying (S8020) power of a predetermined power level to the source device through the connected HDMI; if the sink device determines that the battery state of the source device is above the threshold by detecting that the +5 V signal is received from the source device: transmitting (S8040) a Hot Plug Detect, HPD, signal; transmitting (S2010, S3010) Extended Display Identification Data, EDID, information comprising power supply support information of the sink device to the source device; receiving request power characteristic information of the source device; writing a value related to the sink device being ready to supply the power (S2030, S3030) to the source device in a preparation completion field included in a Status and Control Data Channel Structure, SCDCS, of the sink device; transmitting (S2040, S3040) power supply preparation completion information indicated by the written preparation completion field to the source device; and supplying (S2050, S3050) power to the source device based on the request power characteristic information. connecting to a source device through the HDMI; if the sink device determines (S8010) that a battery state of the source device is below a threshold by detecting that a +5 V signal is not received from the source device, supplying (S8020) power of a predetermined power level to the source device through the connected HDMI; if the sink device determines that the battery state of the source device is above the threshold by detecting that the +5 V signal is received from the source device: transmitting (S8040) a Hot Plug Detect, HPD, signal; transmitting (S2010, S3010) Extended Display Identification Data, EDID, information comprising power supply support information of the sink device to the source device; receiving request power characteristic information of the source device; writing a value related to the sink device being ready to supply the power (S2030, S3030) to the source device in a preparation completion field included in a Status and Control Data Channel Structure, SCDCS, of the sink device; transmitting (S2040, S3040) power supply preparation completion information indicated by the written preparation completion field to the source device; and supplying (S2050, S3050) power to the source device based on the request power characteristic information. transmitting (S8040) a Hot Plug Detect, HPD, signal; transmitting (S2010, S3010) Extended Display Identification Data, EDID, information comprising power supply support information of the sink device to the source device; receiving request power characteristic information of the source device; writing a value related to the sink device being ready to supply the power (S2030, S3030) to the source device in a preparation completion field included in a Status and Control Data Channel Structure, SCDCS, of the sink device; transmitting (S2040, S3040) power supply preparation completion information indicated by the written preparation completion field to the source device; and supplying (S2050, S3050) power to the source device based on the request power characteristic information. <EOS>