A phase detector that compares the phases of data and four-phase first to fourth clocks having a half rate of the data and being 90.degree. out of phase with one another. Exemplary embodiments of the phase detector include first to fourth sampling circuits that sample the data by the four-phase first to fourth clocks; a first comparator that compares sampling data obtained by sampling according to the adjacent two-phase first and second clocks using the first and second sampling circuits, respectively, and when the sampling data is different, outputs a first up signal; and a second comparator that compares sampling data obtained by sampling according to the adjacent two-phase fourth and first clocks using the fourth and first sampling circuits, respectively, and when the sampling data is different, outputs a first down signal.