A computer readable storage medium stores a program that allows a computer to execute a process comprising: acquiring information related to N documents; determining elements of an N×N square matrix D based on the acquired information, in order that D, a positive real number e, and a column vector u having N elements satisfy e u=D u according to the Perron-Frobenius theorem, each of the elements of D being a positive real number; initializing a column vector v having N elements, each of the elements of v corresponding to each of the elements of u; calculating a column vector w=(D v)/|D v|; updating v in the memory to w; iterating the calculating and the updating, until the v satisfies a predetermined condition; and assigning each of elements of the v to the importance of the document.