in a clock signal generating apparatus , an internal clock signal generating circuit generates an internal clock signal . an adjustment data storing section stores an adjusted period designating data and an adjustment content designating data . in this case , the adjusted period designating data indicates ones of n periods of a desired clock signal for adjustment to be performed to obtain the desired clock signal , and adjustment content designating data indicates a content of the adjustment . one period of the desired clock signal is composed of a plurality of periods of the internal clock signal . an adjustment requesting section selectively issues an adjustment request based on the adjusted period designating data and the adjustment content designating data in response to input of desired clock signal . a desired clock signal generating section performs the adjustment in response to the adjustment request while counting pulses of the internal clock signal . as a result , each of specific periods of the n periods designated by the adjusted period designating data is extended or shortened by at least one period of the internal clock signal based on adjustment content designating data . then , the desired clock signal generating section generates desired clock signal based on the adjustment result .