A communication apparatus of an intercom system (1, 2), the communication apparatus being an apparatus that is to be connected with each of first and second devices (100, 200, 300, 400, 500) through a two-wire cable and that is configured to receive a packet signal transmitted from the first device (100, 200, 300, 400, 500) and to transfer the packet signal to the second device (100, 200, 300, 400, 500), the communication apparatus comprising: a receiving section (113, 213, 313, 513) that is configured to receive data in which a preamble and a synchronization pattern are placed in a predetermined position, wherein the receiving section (113, 213, 313, 513) is configured to decode the received data to obtain decoded data, to detect a unique pattern in the preamble and to decode each bit of the received data based on a timing at which the unique pattern is detected; a data reproduction section (133, 233, 333, 533) that is configured to detect the synchronization pattern from the decoded data; and a connection-state detection section (134, 234, 334, 534) that is configured to determine whether the two-wire cable is in normal connection or reverse connection, based on the detected synchronization pattern, a transfer-data processing section (115, 215, 315, 515) that is configured to generate a newly generated preamble, which is a preamble at the time of normal connection, and to generate transfer-data by modulating the decoded data and the newly generated preamble; a transmission section (110, 210, 410, 510) that is configured to transfer the transfer data to the second device (100, 200, 300, 400, 500), characterized in that the transfer-data processing section (115, 215, 315, 515) is configured to generate, when the two-wire cable is in normal connection, the transfer-data by modulating the decoded data and the preamble at the time of normal connection, and the transfer-data processing section (115, 215, 315, 515) is configured to generate, when the two-wire cable is in reverse connection, the transfer-data by modulating the decoded data and the preamble at the time of reverse connection in which a polarity of the preamble at the time of normal connection is inverted, wherein the data reproduction section (133, 233, 333, 533) is configured to perform, when the two-wire cable is in normal connection, post-processing while leaving a data portion of the decoded data unchanged, and the data reproduction section (133, 233, 333, 533) is configured to perform, when the two-wire cable is in reverse connection, the post-processing after inverting the data portion of the decoded data. a receiving section (113, 213, 313, 513) that is configured to receive data in which a preamble and a synchronization pattern are placed in a predetermined position, wherein the receiving section (113, 213, 313, 513) is configured to decode the received data to obtain decoded data, to detect a unique pattern in the preamble and to decode each bit of the received data based on a timing at which the unique pattern is detected; a data reproduction section (133, 233, 333, 533) that is configured to detect the synchronization pattern from the decoded data; and a connection-state detection section (134, 234, 334, 534) that is configured to determine whether the two-wire cable is in normal connection or reverse connection, based on the detected synchronization pattern, a transfer-data processing section (115, 215, 315, 515) that is configured to generate a newly generated preamble, which is a preamble at the time of normal connection, and to generate transfer-data by modulating the decoded data and the newly generated preamble; a transmission section (110, 210, 410, 510) that is configured to transfer the transfer data to the second device (100, 200, 300, 400, 500), the transfer-data processing section (115, 215, 315, 515) is configured to generate, when the two-wire cable is in normal connection, the transfer-data by modulating the decoded data and the preamble at the time of normal connection, and the transfer-data processing section (115, 215, 315, 515) is configured to generate, when the two-wire cable is in reverse connection, the transfer-data by modulating the decoded data and the preamble at the time of reverse connection in which a polarity of the preamble at the time of normal connection is inverted, wherein the data reproduction section (133, 233, 333, 533) is configured to perform, when the two-wire cable is in normal connection, post-processing while leaving a data portion of the decoded data unchanged, and the data reproduction section (133, 233, 333, 533) is configured to perform, when the two-wire cable is in reverse connection, the post-processing after inverting the data portion of the decoded data.