A system for performing a phase control operation includes: an internal clock generation circuit configured to generate an internal clock by delaying a clock by a first delay variation, and generate a reference clock by delaying the clock by a second delay variation, wherein the internal clock generation circuit generates the internal clock by delaying the clock by the first delay variation which is controlled according to a phase difference between the internal clock and the reference clock; and a data input/output circuit configured to input/output data in synchronization with the internal clock.