Observed signals x1(t) to xj(t) from a plurality of sensors disposed in two dimensions are subject to a short-time
Fourier transform, from which signals X1(ω1) to X1(ωN), . . . XJ(ω1) to XJ(ωN) are derived. Using the
independent component analysis process, separation matrices W(ω1) to W(ωN) are produced. Their inverse matrices H(ω1) to H(ωN) are calculated, and for each ωn (n=1, . . . , N), using a pair of elements from each column of H(ωn), Hji(ωn) and Hj′i(ωn), an angle {
circumflex over (θ)}i,jj′(ωn)=cos−1(arg(|Hji(ωn) / Hj′i(ωn)) / (ωnc−1∥dj−dj′∥)) is calculated where arg(α) represents an argument of α, c the propagation velocity of a
signal, and ∥dj−dj′∥ represents a spacing between sensors i and j′. Columns are permuted so that {
circumflex over (θ)}i,jj′(ωn) obtained from each column of H(ω1) to H(ωN) assume an ascending order. For columns which cannot be permuted, the equation ∥qi−dj′∥ / ∥qi−dj∥=|Hji(ωn) / Hj′i(ωn)|=DRi,jj′(ωn) is solved for qi to calculate Ri,jj′(ωn)=∥DRi,jj′(ωn)·(dj−dj′) / (DR2i,jj′(ωn)−1)|. Columns of H(ωn) is permuted so that Ri,jj′(ωn) assume an ascending order. H(ωn) is used to solve the permutation problem of W(ω).