A data transfer system capable of effectively performing serial communication by dividing communication lines into one or plural systems and using the divided lines, according to the number of communication IC's connected on the communication line and a load condition connected to the communication IC is provided. In this system, plural communication lines 410 and 411 are prepared, and the lines 410 and 411 are time divisionally selected according to the contents of connection information set in a select register 462, whereby the serial communication between communication IC's 402 to 405 and a communication controller 401 can be effectively performed.