1. A power-down circuit for reducing operational power consumption of a dynamic MOS integrated circuit during idling conditions, said power-down circuit comprising: a clock divider for generating a divided clock signal in response to an internal clock signal inputted thereto while receiving a control signal, said divided clock signal being synchronized with said internal clock signal and having a repetition rate which is slower than that of said internal clock signal; a control circuit for delivering said control signal when a triggering signal from said dynamic MOS integrated circuit or from an external triggering signal source is inputted thereto; and a clock selecting circuit having a first input for receiving said divided clock signal, a second input for receiving said internal clock signal, a third input for receiving said control signal, and an output coupled to said dynamic MOS integrated circuit, for converting said divided clock signal into a first pair of clock signals and for converting said internal clock signal into a second pair of clock signals, and for selectively transferring to said dynamic MOS integrated circuit either said first pair of clock signals or said second pair of clock signals in response to said control signal, said first pair of clock signals being nonoverlapping with respect to each other and said second pair of clock signals being nonoverlapping with respect to each other.