A connecting device includes an audio signal plug to which an audio signal transmission cable including at least three lines can be connected, the audio signal plug including at least three plug terminals. The three plug terminals of the audio signal plug include first, second, and third plug terminals. The first plug terminal functions as a signal line in which a high-frequency signal is superimposed on an audio signal. The second plug terminal different from the first plug terminal functions as a ground for low-frequency and high-frequency. The third plug terminal different from the first and second plug terminals functions as a ground by capacitive coupling for the high-frequency, and has a function for transmitting a signal different from a signal of the first plug terminal for the low-frequency.