A data output multiplexer for multiplexing and transferring data of a data input/output (I/O) line includes a first latch unit coupled to the data I/O line to latch the data of the data I/O line, a transmission gate unit to transfer an output of the first latch unit in response to control signals, a second latch unit to latch an output of the transmission gate unit, and a leakage current blocking unit to block leakage current between an input node and an output node of the transmission gate unit by fixing the input node and the output node at the same logic level values during a power down mode or a self refresh mode.