PURPOSE: To perform switching without giving any delay to output by passing the output of a square-root extraction operator through a comparator having low-pass filtering characteristic and by switching, based on the output, the output of the operator and the output of an input signal as it is.

CONSTITUTION: The output of the square-root extraction operator EC is compared CP with set voltage Es through the intermediary of the low-pass filter LPF1, and when an input Vi is lower than the set voltage Es, the input signal Vi passing through the low-pass filter LPF2 is outputted Vo through the intermediary of a switch SW, while when the input Vi is higher than the set voltage Es, the input signal Vi passing through the square-root extraction operator EC is outputted Vo through the intermediary of the switch SW.