To provide a communication system which has a large width for available apparatus characteristics to enhance the flexibility in constituting the apparatus.

Upon the receipt of a clock and serial signals from a serial- converting transmitter circuit 220 for generating and sending the serial signals from parallel signals synchronized with the clock, a parallel-conversion receiver circuit 270 restores the clock and the parallel signals. The transmitter circuit 220 has a multiplier 222 for multiplying the frequency of the inputted clock, and the receiver circuit 270 has a divider 272 for dividing the clock outputted from the receiver circuit 270. This makes the same interface circuit (communication system) usable, e.g. in color digital copying machines having different in clock frequency.