PURPOSE: To provide a G3/G4 switching device for an ISDN communication terminal capable of shortening total communication time even when switching is performed by issuing a call from G3 facsimile equipment first, and by performing communication with G4 facsimile equipment of short communication time when first communication fails.

CONSTITUTION: A main control part 11 performs the initial setting of a G3 communication part 13 when the call is issued to an opposite terminal whose communication protocol is unknown, and sends out a call setting message, and attaches a sub address as a dummy on the call setting message by a LAPD controller 14. Thence, the response of the opposite terminal for the call setting message of the G3 facsimile equipment on which the sub address is attached is monitored by the LAPD controller 14, and when no response is returned, shift from the G3 communication part to a G4 communication part 12 is performed, and the call setting message without the sub address is sent out.