PURPOSE: To prevent erroneous connection of a cable, by a method wherein a mutual short-circuiting is caused between specified lines of a plurality of cables to which first and second circuit boards are connected to form one reciprocation line, a voltage of which is monitored.

CONSTITUTION: Circuit boards 1 and 2 are connected through cable bundles 3 and 4 comprising a plurality of cables. A short-circuiting is caused by a short- circuiting means 11 between specified lines of the cable bundles 3 and 4 on the side of the circuit board 1. Moreover, one open end of the line is connected to a power source on the second circuit board through a resistor 21 and the other open end thereof is earthed through a resistor 22 while being connected to a voltage comparator 25. A voltage divided with these resistors is monitored thereby enabling accurate detection of erroneous connection of the cables.