A communication system is the communication system in which an I2C slave 10 and an I2C master 30 are connected by a serial clock SCL configured to convey a clock transmitted from the I2C master 30 and a serial data SDA configured to bidirectionally transmit data, and includes: a first signal line 41 configuring a portion of the serial data SDA and configured to transmit predetermined data from the I2C master 30 to the I2C slave 10 a second signal line 42 configuring a portion of the serial data SDA and configured to transmit predetermined data from the I2C slave 10 to the I2C master 30 a first signal line selection portion 51 and a second signal line selection portion 52 configured to select either one of the first signal line and the second signal line so as to be effective as the serial data SDA and a signal control portion 31 configured to control a selecting operation in the signal line selection portion.
