A facsimile apparatus, to which an external telephone enables to be connected through a line, has a line I/F section that transfers and receives signal through a network, a voltage detection section that is provided in a semiconductor DAA and detects a line voltage of the line, an adjustment section that adjusts a line voltage during establishment of a connection through the line so as to be within a predetermined voltage, and an external telephone monitor section that acquires a value of the line voltage detected by the voltage detection section at regular time intervals and monitors a use state of the external telephone based on the acquired values of the line voltage.