An internal clock generating circuit of a semiconductor device includes: a delay chain having a plurality of delay units for generating multi-phase clocks by adjusting an input clock; a thermometer for outputting a thermometer code value in response to an input selection data; a multiplexer for selectively outputting one of a plurality of clocks input from the delay chain in response to the thermometer code value of the thermometer; and a pulse regenerator for outputting an adjusted internal clock by restoring a pulse form of the clock output from the multiplexer into its original state and controlling the delay thereof as much as desired.