PURPOSE: To decide a multiple factor in a very short time by obtaining a ratio of a threshold level to a digital input signal, and obtaining a sum of a product between the ratio and a 1st coefficient, a product between the squared ratio and a 2nd coefficient, and '1'.

CONSTITUTION: An input level 8J is fed to a level comparator 26, in which the level is compared with a predetermined threshold level eth, and when the input level ei is less than the level eth, '1'is fed to a multiplier 22 as a multiple (g). When the input level ei is larger than the level eth, the level ei is fed to a divider 28 as it is. The threshold level eth is fed to the divider 28, and the divider 28 divides the threshold level eth by the input level ei to calculate a quotient eth/ei. The quotient eth/ei is fed to a computing element 30, in which the calculation of x=(1-eth/ei) is calculated. The value (x) is fed to a computing element 32, in which g=(1+αx+βx2) is calculated. The multiple (g) obtained in this way is fed to a multiplier 22, in which the multiple (g) is multiplied with the digital input signal fed from an input terminal 20 to compand the digital input signal.