PURPOSE: To provide a filter computing device, which prevents the occurrence of deterioration in data precision of input signals, for a fixed point computing type digital signal processing device that is incorporated into the audio signal encoding device of a digital mobile communication, etc.

CONSTITUTION: Reflection coefficients K (i) are obtained from LPC coefficients A (i) by a reflection coefficient computing means 11. Then, a normalized predictive error power δ2 is computed from the coefficients K (i) by a normalized predictive error power computing means 12. Next, employing the δ2, the data shift number SI in a filter computation is determined by a shift number computing means 13. Then, an inverse filter computing means 14 performs an inverse filter computation employing the number SI, an input signal X (n) and the coefficients A (i) to obtains an output signal Y (n).