PURPOSE: To completely prevent damage, breakage or deterioration of a load, a connector, etc., by eliminating outputting of an output voltage when a plug-in power source module is mismounted.

CONSTITUTION: When plug-in power source modules 1-3, 1-2 are mounted and power source module side connectors 4-1, 4-2 are engaged with printed circuit board side connectors 6-1, 6-2, the fact that a circuit element having patterns C, D of the module 1-3 does not match a circuit element having a pattern G of a printed circuit board 5 is detected by a discriminator 3, and a power source ON inhibit signal is output. Thus, since an output voltage is not output, damage, breakage or deterioration of a load 7-1, the connectors 4-1, 6-1, etc., can be completely prevented.