A semiconductor device includes a comparison unit configured to compare the phases of a plurality of clocks having different frequencies and output a phase comparison signal, a phase inversion control unit configured to generate a phase inversion control signal, and a start control unit configured to generate a start control signal in response to a clock enable signal, wherein the comparison unit is configured to start an operation in response to the start control signal and invert, in response to the phase inversion control signal, a phase of an internal clock generated from one of the plurality of clocks when the plurality of clocks have different phases.