A connection method of connecting a feeding unit (20, 20C) provided with a power supply circuit with a connector unit (30, 30C) provided with a connector to be connected to a user device, wherein a connector unit (30, 30C) is suitable for a feeding system (10, 10A-D) in which a feeding unit (20, 20C) and the connector unit (30, 30C) are connected by a cable (40, 40A-D), wherein the connector unit (30, 30C) is provided with three or more specific ports which comprise at least one power supply port and at least one ground port and which are configured to respectively correspond to three or more specific ports included in the feeding unit (20, 20C) and comprising at least one power supply port and at least one ground port, wherein in each of the feeding unit (20, 20C) and the connector unit (30, 30C), plural ports having same function which are included in the specific ports (261, 321) are connected to each other; the connector is a connector conformable to Universal Serial Bus standards; and the method comprising, in at least one of a connection of the power supply port of the feeding unit (20, 20C) and the power supply port of the connector unit (30, 30C) and a connection of the ground port of the feeding unit (20, 20C) and the ground port of the connector unit (30, 30C), adjusting a voltage drop between the feeding unit (20, 20C) and the connector unit (30, 30C) by changing number of parallel lines in at least a partial section according to a length of the connection, wherein the connector unit (30, 30C) further comprises a communication port; the communication port of the connector unit (30, 30C) is connected to the connector in the connector unit (30, 30C); and the communication port of the connector unit (30, 30C) is for being connected to a communication port provided in the feeding unit (20, 20C) to carry out communication conformable to the USB standards. a connector unit (30, 30C) is suitable for a feeding system (10, 10A-D) in which a feeding unit (20, 20C) and the connector unit (30, 30C) are connected by a cable (40, 40A-D), wherein the connector unit (30, 30C) is provided with three or more specific ports which comprise at least one power supply port and at least one ground port and which are configured to respectively correspond to three or more specific ports included in the feeding unit (20, 20C) and comprising at least one power supply port and at least one ground port, wherein in each of the feeding unit (20, 20C) and the connector unit (30, 30C), plural ports having same function which are included in the specific ports (261, 321) are connected to each other; the connector is a connector conformable to Universal Serial Bus standards; and the method comprising, in at least one of a connection of the power supply port of the feeding unit (20, 20C) and the power supply port of the connector unit (30, 30C) and a connection of the ground port of the feeding unit (20, 20C) and the ground port of the connector unit (30, 30C), adjusting a voltage drop between the feeding unit (20, 20C) and the connector unit (30, 30C) by changing number of parallel lines in at least a partial section according to a length of the connection, wherein the connector unit (30, 30C) further comprises a communication port; the communication port of the connector unit (30, 30C) is connected to the connector in the connector unit (30, 30C); and the communication port of the connector unit (30, 30C) is for being connected to a communication port provided in the feeding unit (20, 20C) to carry out communication conformable to the USB standards.