PURPOSE: To prevent the reversal of motor exactly and easily by sequentially connecting respective apparatuses to each other in such a way that the phase order of inputs among the units which contain motors are the same and by connecting a negative-phase detector to the line of power supply.

CONSTITUTION: Three-phase AC line is sequentially connected to each other from the panel board 1 through the controller 6, for example, the magnetic disc units from 3∼1 to 3-N are connected to the controller 6 and to each other with cables which have connectors at both ends, the cables are connected to the connectors in such a way that the phase order for each motor is the same. The phase order of three-phase AC is checked by the negative-phase detector 2 set in the controller 6, the motors of 5∼1 to 5-N are turned in the normal direction when the phase is positive, the relays of 4∼1 to 4-N are turned off by the contact rl 1 to inhibit the operation of motors when the phase is negative. By this, the reversal of motor can be prevented even if the units are shifted since the phase order is exactly and easily connected to each other.