Abstract: A command conversion device 14 is connected between an amplifier device 11 and a portable player 13. The amplifier device 11 sends/receives a plurality of types of first commands corresponding to a plurality of types of devices (e.g., CD players, MD recorders, tape recorders, etc.), and performs an operation based on a received first command. The portable player 13 sends/receives a second command, and performs an operation based on a received second command. The command conversion device 14 includes a selector for selecting a type of a first command, and a converter. When a first command of the type selected by the selector is received from the amplifier device 11, the converter converts the received first command into a second command, and sends the second command to the portable player 13.