A reversible unit (10) is attached by being mounted over a pair of adjacently disposed electromagnetic contactors (1a) and (1b). The reversible unit (10) includes a pair of interlock plates (11) and (12), a lock piece (13), and a unit case (10b) accommodating the pair of interlock plates (11) and (12) and the lock piece (13). The lock piece (13) rotates in a first direction with the movement of one of the interlock plates (12) in a turn-on operation of one of the electromagnetic contactors (1a) to prevent the movement of the other interlock plate (11), and rotates in a second direction different from the first direction with the movement of the other interlock plate (11) in a turn-on operation of the other electromagnetic contactor (1b) to prevent the movement of one of the interlock plates (12). The unit case (10b) is provided with a lock piece display window (10g) which allows determination of the rotation of the lock piece (13) in the first direction or the second direction.