The expansion method in the kinetic theory of gases is often used for the calculation of entropy production in rarefied gas flows, usually in terms of Her-mite polynomials. In this paper, the method of Sonine polynomials expansion is used and the calculation of entropy production is carried out. First, the formal evaluation of the entropy production in rarefied gas flows is introduced. Then the calculation of the entropy production for arbitrary molecules with spherically symmetric intermolecular forces by Sonine polynomials expansion is presented and the formula for entropy production in the thirteen-moment approximation is obtained. It is shown that the entropy production can be expressed as a linear combination of square terms in the thirteen-moment approximation. This formula can be applied to slightly rarefied gas flows.