A communication system, which comprises a first communication interface device for a first air conditioning unit and a second communication interface device for a second air conditioning unit, wherein each communication interface device comprises: a sampler that is adapted to sample a receive signal received via a communication line; a data transfer rate acquirer that is adapted to acquire a data transfer rate; a bit determiner that is adapted to make a bit determination regarding the receive signal at the acquired data transfer rate acquired by the data transfer rate acquirer, to thereby acquire serial data contained in the receive signal; a transmitter that is adapted to send, via a communication line a transmit signal containing serial data sent by the processor; a processor communicator that is adapted to output to the transmitter the serial data sent by the processor, and output to the processor the serial data acquired by the bit determiner; and an adjuster that is adapted to, when the transmitter sends the transmit signal multiple times at different data transfer rates, adjust the data transfer rate based on the receive status of respective response signals thereto, characterized in that the adjuster of each communication interface device is adapted to: if the receive signal is successfully received at the acquired data transfer rate acquired by the data transfer rate acquirer, cause the transmitter to send the transmit signal at a greater data transmit rate than the acquired data transfer rate; and if, after the transmission of the transmit signal, the receive signal is not received and a timeout has occurred, cause the transmitter to resend a definitive signal at a lesser data transfer rate than the acquired data transfer rate, and then if a response signal to the definitive signal is successfully received, set a current data transfer rate as a data transfer rate to be used in transmission. a sampler that is adapted to sample a receive signal received via a communication line; a data transfer rate acquirer that is adapted to acquire a data transfer rate; a bit determiner that is adapted to make a bit determination regarding the receive signal at the acquired data transfer rate acquired by the data transfer rate acquirer, to thereby acquire serial data contained in the receive signal; a transmitter that is adapted to send, via a communication line a transmit signal containing serial data sent by the processor; a processor communicator that is adapted to output to the transmitter the serial data sent by the processor, and output to the processor the serial data acquired by the bit determiner; and an adjuster that is adapted to, when the transmitter sends the transmit signal multiple times at different data transfer rates, adjust the data transfer rate based on the receive status of respective response signals thereto, characterized in that the adjuster of each communication interface device is adapted to: if the receive signal is successfully received at the acquired data transfer rate acquired by the data transfer rate acquirer, cause the transmitter to send the transmit signal at a greater data transmit rate than the acquired data transfer rate; and if, after the transmission of the transmit signal, the receive signal is not received and a timeout has occurred, cause the transmitter to resend a definitive signal at a lesser data transfer rate than the acquired data transfer rate, and then if a response signal to the definitive signal is successfully received, set a current data transfer rate as a data transfer rate to be used in transmission. if the receive signal is successfully received at the acquired data transfer rate acquired by the data transfer rate acquirer, cause the transmitter to send the transmit signal at a greater data transmit rate than the acquired data transfer rate; and if, after the transmission of the transmit signal, the receive signal is not received and a timeout has occurred, cause the transmitter to resend a definitive signal at a lesser data transfer rate than the acquired data transfer rate, and then if a response signal to the definitive signal is successfully received, set a current data transfer rate as a data transfer rate to be used in transmission.