A clock signal generating section generates a clock signal of a predetermined frequency. A frequency spreading section spreads the frequency of the clock signal generated by the clock signal generating section based on a predetermined spread ratio. A controlling section sets the spread ratio of the frequency spreading section in accordance with a used state of a serial communication device and a parallel communication device. The clock signal controlling device having the above arrangement enables to suppress an electromagnetic wave emitted from an electronic device, and to keep a computation processing speed in the electronic device, and an operation speed of the electronic device from unduly lowering.