A duty cycle correcting circuit includes a first duty ratio correcting unit that widens a high-level period of an input clock in response to a detection signal, thereby correcting a duty ratio of the input clock to output a first corrected clock. A second duty ratio correcting unit narrows the high-level period of the input clock in response to the detection signal, thereby correcting the duty ratio of the input clock to output a second corrected clock. A clock selecting unit selectively outputs the first corrected clock or the second corrected clock as an output clock in response to the detection signal. A duty ratio detecting unit detects a duty ratio of the output clock, thereby generating the detection signal.