PURPOSE: To simplify the line changing job with simple constitution by adopting the constitution that a 1st communication line is connected in common to plural 2nd communication lines, and the connecting point is subject to selective switching control in interlocking with the switching of the switching operation section.

CONSTITUTION: When, for example, a 1st internal communication line 13 is faulty in the line configuration comprising a 1st external communication line 11 and the 1st internal communication line 13, at first an A changeover operating section 20 is operated to turn off a 1st switch 16. Then a B changeover operating section 21 corresponding to the 2nd internal communication line 14 not in use is operated to turn on a 2nd switch 17. Thus, the external communication line 11 is changed to the internal communication line 14 not in use and the communication is executed successively. Since the line changeover is constituted in interlocking with the selecting operation of the A∼D changeover operating sections 20∼23, the changeover job is improved as much as possible, and the mis-operation due to mis-connection is prevented surely.