A clock frequency divider circuit in accordance with the present invention is capable of generating a clock signal that makes it possible to perform an expected proper communication operation in communication with a circuit operating by a clock having a different frequency, and includes a mask control circuit 20 and a mask circuit 10. The mask control circuit 20 includes a mask timing signal generation circuit 22 that generates a mask timing signal 29 used to preferentially mask a clock pulse at a timing other than communication timings among M clock pulses of the input clock signal based on a communication timing signal 26, and a mask restraint circuit 62 that carries out a process to restrain masking of a clock pulse at a communication timing. The mask circuit 10 generates an output clock signal by masking clock pulses of an input clock signal according to a mask signal 50 generated by the mask control circuit.