A frequency dividing circuit divides a master clock frequency by a non-integer factor to provide an output clock signal whose frequency is equal to the frequency of the master clock signal divided by that non-integer factor. Forward mapping refers to mapping of fragments from a reticle layout to a target layout, while backward mapping refers to mapping of fragments from the target layout to the reticle layout. A capacity of the retention tank section 60 is set to have a capacity greater than that of the circulating tank section 58.