PURPOSE: To set properly a signal level at data communication depending on a line current.

CONSTITUTION: A CPU 3a of a microcomputer 3 in the standby state detects the arrival of a call, then the CPU 3a throws a CML relay 7 from the position of a slave telephone set 1 to the position of a facsimile equipment. When a hook-off signal is received from a current sensor 6 attended with hook-off of a handset of the slave telephone set 1, a digital signal converted from the current signal from the current sensor 6 by an A/D converter 5 (converted current) is read and a gain corresponding to the converted current (line current) is set to a pre-gain amplifier 9a of a MODEM 9.