A method of and an apparatus for detecting the all make busy state of a line concentrator or an exchange in which all outgoing circuits are divided into a plurality of groups, at least one circuit is specified from each of the groups, the specified circuits are successively scanned with a scanning signal producing therefrom a plurality of phase pulse signals having phase ranges resulting from dividing phase ranges by at least the number of the specified circuits of the groups corresponding to the latter phase ranges which are one period of the scanning signal divided by the number of the groups, the busy state of the specified circuit of a group is detected to decide the busy state of the group, and the busy states of all groups at one cycle of the scanning are detected to detect the all make busy state of the line concentrator or the exchange.
