A printed circuit board loaded with an electronic circuit and constructed to be capable of being inserted in and withdrawn from a bus line on on-line status, in which the input to or the output from the printed circuit board is locked out in response to the presence of two conditions, one is the presence of a demand for insertion or withdrawal of the printed circuit board on on-line status and the other is the presence of such a condition that no information exchange is being made between the printed circuit board and other unit connected with the bus line, thereby releasing the electrical connection between the bus line and the printed circuit board so that the printed circuit board can now be inserted in or withdrawn from the bus line on on-line status.