A data communication system has a first data communication circuit for outputting a clock signal to a clock signal line, receiving data input from a data signal line, and outputting data as open drain output to the data signal line, a second data communication circuit for receiving input of a clock signal from the clock signal line, receiving input of data from the data signal line, and outputting data as open drain output to the data signal line, a first pull-up resistor connected between the data signal line and the wiring of a power supply potential, a second pull-up resistor for selectively pulling up the data signal line, and a pull-up control circuit that is connected to the second pull-up resistor, and strengthens pull-up of the data signal line at least in response to a clock signal.