To achieve data communication with no error without using a dedicated line when attaching later security equipment such as a sensor and equipment for issuing an alarm.

The intercom system includes an entrance slave unit 2 installed at the entrance; an intercom master unit 1A installed in a house for informing a user of calling from the entrance slave unit 2 and responding the calling to establish a telephone call; a sensor terminal 5 for detecting a security failure occurring in the house; and an additional speaker 4 for noticing the calling from the entrance slave unit 2 noticing as an alarm the security failure detected by the sensor terminal 5. The additional speaker 4 is connected with the intercom master unit 1A via a power line L2 for performing power line carrier wave communication between it and the intercom master unit 1A.