1. A non-transitory computer readable storage medium storing a document importance calculation 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, each of the elements of u corresponding to an importance of each of the N documents; initializing a column vector v having N elements so as to store v in a memory, 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 of w and the updating of v, until the updated v satisfies a predetermined condition, so as to obtain the updated v being an approximation of u; and assigning each of elements of the updated v to the importance of a corresponding document.