A transferring method from an ISDN telephone to an ISDN data terminal comprises the steps of: transmitting a message to request a temporary interruption of a call to the ISDN from the call received ISDN telephone; transmitting a message to request a start of the interruption call to the ISDN from the ISDN data terminal connected to the same bus, as that of the ISDN telephone; and executing a data communication such as a G3 facsimile communication by the ISDN data terminal.