To facilitate the decomposition processing like LU decomposition or LDL decomposition of a matrix.

When a matrix to be decomposed is an asymmetrical band matrix of (n) rows and (n) columns which has an upper band width bwu and a lower band width bwl, matrix elements in the band region are compressed into a form of a two-dimensional storage array consisting of (bwn+bwl*2+1) rows and (n) columns, and they are inputted to a parallel computer system. At this time, each row of the asymmetrical band matrix is stored on a right ascending line inclined at 45° in the storage array. A compiler automatically distributes and arranges compressed and inputted matrix data in plural processors to constitute a two-dimensional processor grid where processors having elements of one row/column of the original matrix are arranged on a line. Each processor refers to this two-dimensional processor lattice to transfer required data and updates matrix elements which this processor takes charge of.