PURPOSE: To detect automatically an unmounted printed board, by discriminating whether the printing board is mounted or not on a mother board by an address matching means.

CONSTITUTION: A controlling part 1 for an electronic exchange or the like is put on an unshown mother board. The printed board 2 forming a trunk is connected to the controlling part 1 by an unshown connector. An address setting switch group 4 and a comparator 5 for address matching are arranged on the printed board 2. When an address signal is sent from the controlling part 1 to the printed board 2 through a terminal 3 in the shown state, the comparator 5 compares the sent signal with an address set up by itself. When both addresses coincide with each other, a signal L indicating the mounting to the printed board 2 is returned to said controlling part 1 through an inverter 6 and a terminal 7.