A coding apparatus capable of coding a spectrum at a low bit rate and with high quality without producing any disturbance in a harmonic structure of the spectrum. In this apparatus, internal state setting section (106) sets an internal state of filtering section (107) using first spectrum S1(k). Pitch coefficient setting section (109) outputs pitch coefficient T by gradually changing it. Filtering section (107) calculates estimated value S′2(k) of second spectrum S2(k) based on pitch coefficient T. Search section (108) calculates the degree of similarity between S2(k) and S′2(k). At this time, pitch coefficient T′ corresponding to the maximum calculated degree of similarity is given to filter coefficient calculation section (110). Filter coefficient calculation section (110) determines filter coefficient βi using this pitch coefficient T′.