An intercom system, wherein a master device is connected with a slave device through a two-wire cable, and a packet signal is transmitted and received between the master device and the slave device by time division duplex, the slave device includes a camera, a microphone, and a speaker, transmits an uplink packet signal which includes image data obtained by the camera and sound data obtained by the microphone, to the master device, receives a downlink packet signal from the master device so as to reproduce data, and outputs sound data included in the downlink packet signal from the speaker, the master device includes a display, a microphone, and a speaker, receives the uplink packet signal from the slave device so as to reproduce data, displays image data included in the uplink packet signal in the display, outputs sound data included in the uplink packet signal from the speaker, and transmits the downlink packet signal which includes sound data obtained by the microphone, to the slave device, characterized in that the slave device determines whether the two-wire cable is forward connected or reversely connected by checking for a predetermined unique preamble pattern in the received downlink packet signal based on the number of clocks of a measurement section which is positioned between adjacent falling edges or between adjacent rising edges, and in case the predetermined unique preamble pattern is detected, checking a synchronization pattern in the received downlink packet signal, wherein the slave device stores a known first synchronization pattern for checking whether the two-wire is forward connected and a second synchronization pattern which is obtained by reversing the first synchronization pattern, and in a case where the synchronization pattern detected from the received downlink packet signal coincides with the first synchronization pattern, the slave device determines that the two-wire cable is forward connected, and in a case where the detected synchronization pattern coincides with the second synchronization pattern, the slave device determines that the two-wire cable is reversely connected, and in a case where the two-wire cable is reversely connected, the slave device reproduces data after reversing the received downlink packet signal, and transmits the uplink packet signal after reversing the uplink packet signal, and even when the two-wire cable is reversely connected, the master device reproduces data without reversing the uplink packet signal, and transmits the downlink packet signal without reversing the downlink packet signal. checking for a predetermined unique preamble pattern in the received downlink packet signal based on the number of clocks of a measurement section which is positioned between adjacent falling edges or between adjacent rising edges, and in case the predetermined unique preamble pattern is detected, checking a synchronization pattern in the received downlink packet signal, wherein the slave device stores a known first synchronization pattern for checking whether the two-wire is forward connected and a second synchronization pattern which is obtained by reversing the first synchronization pattern, and in a case where the synchronization pattern detected from the received downlink packet signal coincides with the first synchronization pattern, the slave device determines that the two-wire cable is forward connected, and in a case where the detected synchronization pattern coincides with the second synchronization pattern, the slave device determines that the two-wire cable is reversely connected, and in a case where the synchronization pattern detected from the received downlink packet signal coincides with the first synchronization pattern, the slave device determines that the two-wire cable is forward connected, and in a case where the detected synchronization pattern coincides with the second synchronization pattern, the slave device determines that the two-wire cable is reversely connected, and