1. A data extraction method comprising: storing a plurality of objects to be processed in a memory device, each of the objects including a single set of two data entities which differ in attribute from each other; converting each of the objects stored in the memory device to one of elements arranged in a matrix, by setting one of the two data entities at a row of the matrix and the other at a column of the matrix; extracting from the matrix a submatrix which is generated by replacing rows and columns of the matrix and is dense with the objects, the submatrix being configured by a part of the matrix.