A speech coder and so forth for preventing deterioration of the quality of a reproduced speech signal while reducing the coding rate. In a speech signal modifying section (101) of the coder, a masking threshold calculating section (114) calculates a masking threshold M(f)) of the spectrum S(f) of an input speech signal, an ACB sound source model spectrum calculating section (117) calculates an adaptive codebook sound source model spectrum SACB(f), an input spectrum shape modifying section (112) refers to both values of the masking threshold M(f) and the adaptive code book sound source model spectrum S′ACB(f) having an LPC spectral envelope and carries out a preprocessing of the spectrum S(f) so that the shape of the spectrum S(f) is modified to match a CELP coding section (102) of the succeeding stage. The CELP coding section (102) carries out CELP coding of the preprocessed speech signal and outputs a coded parameter.