A method of demanding/supplying a necessary electric power through a self-adjusting process between two devices connected by a Universal Serial Bus (USB), removing the requirement of disconnecting and re-connecting cable lines in order to make a power demand again. A power supplying device first determines how much electric power is demanded by a demanding device, then sends the device a signal of impossible supply or the like if the demand can not be met. The supplying device also determines whether or not a new electric power demand can be supplied if the demanding device demands again, and the supplying device then supplies power to the demanding device if the demand can be met. A power demanding device demands an electric power from a supplying device connected through the USB. The connected power demanding device first reduces the demand if a signal of impossible supply or the like is received from the supplying device, then demands the reduced amount from the supplying device.