A slew rate adjusting circuit includes: a variable delay unit configured to delay an output of a signal source in response to a first control signal; an output buffer unit configured to adjust a slew rate in response to a second control signal; a differential buffer unit configured to compare the resulting signal output from the output buffer unit with a reference voltage; a first phase comparison unit configured to compare a phase of the output of the signal source with a phase of a signal; a first count unit configured to count the signal output from the first phase comparison unit and output the first control signal; and a second count unit configured to count the signal output from the second phase comparison unit and output the second control signal based on a count result.