PURPOSE: To exactly check all connecting lines of a connector even if a pull-up resistance (or a pull-down resistance) is used on an output side of a processor or an input side of a unit, in the method for detecting whether the connection of the processor and each extension unit is correct or not.

CONSTITUTION: This method is constituted so that two fixed data whose logic is inverted each other are stored in advance in a unit 2, the two data concerned designated by two addresses whose logic is inverted each other from a processor 1 are read out of the unit concerned 2, and it is decided that the connecting state is normal, when there is no error in the two data concerned.