A digital circuit system includes: a register, for receiving and registering digital data; an operation unit, for operating and generating resulting data according to the digital data registered in the first registering unit; a second register, for receiving and registering the resulting data; a multi-phase clock signal generating unit, for generating a plurality of reference clock signals having different phases with each other; a first selector, for selecting one of the reference clock signals to output a first clock signal to the first registering unit; and a second selector, for selecting another of the reference clock signals to output a second clock signal to the second registering unit.