An encoder, decoder, encoding method, and decoding method enabling acquisition of high-quality decoded signal in scalable encoding of an original signal in first and second layers even if the second or upper layer section performs low bit-rate encoding. In the encoder, a spectrum residue shape codebook stores candidates of spectrum residue shape vectors, a spectrum residue gain codebook stores candidates of spectrum residue gains, and a spectrum residue shape vector and a spectrum residue gain are sequentially outputted from the candidates according to the instruction from a search section. A multiplier multiplies a candidate of the spectrum residue shape vector by a candidate of the spectrum residue gain and outputs the result to a filtering section. The filtering section performs filtering by using a pitch filter internal state set by a filter state setting section, a lag T outputted by a lag setting section, and a spectrum residue shape vector which has undergone gain adjustment.