PROBLEM TO BE SOLVED: To provide a technique for improving precision of signal processing with a small increase in computational complexity.SOLUTION: An expansion adaptive smoothing LSP linear conversion section 328 defines "p" as an integer equal to or larger than 1 and ω[1], ω[2], ..., ω[p] as a frequency region parameter string deriving from a linear prediction coefficient string corresponding to a signal of a time region when a coefficient string in a frequency region in which intervals of frequencies between adjacent sample points corresponding to a sound signal in a predetermined time section are irregular is considered to be regular in intervals of frequencies at the sample points, accepts, as an input, the frequency region parameter string of ω[1], ω[2], ..., ω[p], and finds values of frequency region parameters ∼ω[i] after conversion through linear conversion of ∼ω[i] (i=1, 2, ..., p) in a frequency region parameter string of ∼ω[1], ∼ω[2], ..., ∼ω[p] after conversion based upon r
