A semiconductor memory device including a first clock transmission path configured to receive a source clock swinging at a CML level through a clock transmission line in response to an enable signal, and to convert the source clock into a clock swinging at a CMOS level. The device also includes a second clock transmission path configured to convert the source clock in a clock swinging at a CMOS level in response to the enable signal, and to output the converted clock through the clock transmission line and a data output unit configured to output data in response to output clocks of the first and second clock transmission lines.