According to one embodiment, there is provided an instruction method for providing an instruction from a supply device to a processing device via an HDMI cable, the method comprising detecting a signal from an operating switch in the supply device, generating a packet signal in which instruction information according to the detected signal is described, in an ASCII description area of source product description (SPD) of a packet signal for use in the HDMI standard, in the supply device, supplying the packet signal from the supply device to the processing device via the HDMI cable, and detecting the instruction information from the ASCII description area of SPD of the packet signal in the supply device, and performing a desired operation accordingly.