To provide an interphone control system that can suppress the power consumption under a constant current source of a master unit to be within a prescribed range even when the length of a cable changes depending on installation conditions of the master unit and a slave unit in the interphone control system.

In the interphone control system where the master unit 110 and the slave unit 400 are interconnected by a cable, the master unit 110 converts a constant voltage source 100 therein into a constant current source 200 and the master unit 110 supplies a current to the slave unit 400, the master unit 110 applies feedback control to a voltage changed with the wire resistance of the cable to the constant voltage source 100 in the master unit 110, converts an output voltage Va of the voltage source 100 subjected to the feedback control into the constant current source 200, which supplies a constant current to the slave unit 400.