An input buffer includes a first buffer circuit to amplify a difference between a first input signal and a second input signal; a second buffer circuit formed of a replica circuit of the first buffer circuit to generate a common mode output signal in response to the first input signal; and a detector to compare the common mode output signal with a reference output signal and to control the first and second buffer circuits according to the comparison result such that a level of the common mode output signal coincides with a level of the reference output signal.