PURPOSE: To prevent a package from being erroneously inserted by setting a package ID (IDentification number) specific to each slot in an ID setting circuit, and by using an ID discriminating circuit and a pilot lamp to turn on the auxiliary power supply before turning on the main power supply.

CONSTITUTION: An ID setting circuit 6 for setting ID specifically for package 5 is used to set an ID specific to each slot. In the package 5, an ID signal 8 from slot 2 decides whether the package 5 is mounted to a predetermined slot, and if the position of slot 2 is correct, a pilot lamp 11 is lit by a lighting signal from an IC discriminating circuit 9. As a result, it is judged that the position in which package 5 has been inserted is correct.