It is well known that the Onsager reciprocal relations give important identities between transport coefficients for heat and charge conduction in the linear response regime. For example, the Peltier coefficient is related to the Seebeck coefficient S in a simple form as S 1⁄4 =T , where T is the temperature. The Onsager–Casimir symmetry, which is the extension of the Onsager relation in the presence of an external magnetic field, is also derived from the microscopic reversibility. Recently, a general relation called the steadystate fluctuation theorem has been derived from the same concept of the microscopic reversibility. It gives an identity equation on the probability of the entropy production S during time as