PURPOSE: To obtain the low pass filter obtaining a coefficient with high accuracy from a short coefficient word length.

CONSTITUTION: Equations of A0=a0.δ1, A1=a1.δ1, A2=a2.δ1, B1=δ2 (b1-2) and B2=δ2 (b2+1) are established, where a0, a1, a2 are input coefficients, b1, b2 are output feedback coefficients and δ1, δ2 are multiple of coefficients. An input signal Xn, data Xn-1 of one preceding sample obtained by delay devices 1, 2, and data Xn-2 of two preceding sample are multiplied respectively by A0, A1, A2 at coefficient devices 10, 11, 12 respectively and they are added by an adder 43 and multiplied by 1/δ1 at a coefficient device 31 and the result is fed to a synthesizer 46. data Yn-1 of one preceding sample and data Yn-2 of two preceding sample obtained by delay devices 3, 4 from an output Yn are multiplied respectively by B1, B2 at coefficient devices 21, 22 and added by an adder 44 and multiplied by 1/δ1 at a coefficient device 32 and the result is fed to the synthesizer 46. Then the data Yn-1 of one preceding sample from the output Yn obtained by the delay devices 3, 5 is doubled by a coefficient device 35 and fed to the synthesizer 46 together with data Xn-2 of two preceding sample obtained by the delay device 4 to provide an output signal Yn.