The present invention provides: a dopant solution for an electroconductive polymer characterized in that it comprises at least one selected from the group consisting of alkylamine salts and imidazole salts of benzene skeleton sulfonic acids and naphthalene skeleton sulfonic acids, having at least one OH group and at least one sulfonate group, at a concentration of 40 mass % or more; an oxidant and dopant solution for an electroconductive polymer including a mixture of the organic salt of the dopant as mentioned above, and a persulfate organic salt as an oxidant; an electroconductive composition including an electroconductive polymer prepared by using the oxidant and dopant solution as mentioned above; and solid electrolytic capacitor using the electroconductive composition as a solid electrolyte. The electroconductive composition has an improved electric conductivity, and the solid electrolytic capacitor has an improved reliability for a long time.