When a fluid flows through a porous medium, charges in the electric double layer are transported, resulting in a streaming current. This is the representative mechanism of self-potential widely observed in the field. In the conventional Helmholtz-Smoluchowski relation, the streaming current density (is) is represented as is = Ccgrad Pp, where Cc is the streaming current coefficient and Pp is the fluid pressure. Cc depends on the hydraulic property of the rock and the electrochemical property called the zeta potential. It was shown that the streaming current coefficient was clearly affected by the flow state: it was constant at a low Reynolds number Re, but decreased at Re > 1--10. This is a mechanical effect (i.e., smaller apparent permeability for nonlinear laminar flow), not a chemical effect on the zeta potential. The mutually contradicting values of zeta potential, which is calculated from measured Cc, obtained in different studies may be due to this effect. In addition, we found that the decreasing rate of Cc with increasing Re is much less than that of permeability. This means that nonlinear laminar flow induces more streaming current per unit flow rate.