A method of controlling signal lines among a number of audio/video units in a system where a central unit including a selector and the other audio/video units are mutually connected via a single control bus and also via audio/video signal cables includes the steps of enabling the central unit to manage identification numbers of the audio/video units on the control bus and also connection data among the audio/video units, classifying the audio/video units in accordance with the respective categories, connecting the signal line between one of the audio/video units and another one of the units, enabling one audio/video unit to designate by category the other audio/video unit to be connected and, after such designation, enabling the central unit to select the specific audio/video unit that satisfies the requirement, out of all of the audio/video units of the designated category, and then switching the selector of the central unit thereby connecting the desired signal line between the relevant audio/video units.