A semiconductor device includes a clock generator which receives an input clock and generates an output clock, a reference voltage generator which receives the input clock or the output clock, generates a sub-reference voltage in accordance with a frequency of the input clock or a frequency of the output clock, and generates a reference voltage using the sub-reference voltage and a preset error voltage, and a clock detector which receives the output clock, generates a first output voltage in accordance with the output clock, and compares the generated first output voltage with the reference voltage to output an error signal based on the output clock, wherein the preset error voltage is set in accordance with a degree of preset error of the output clock.