A mixture solution for preparing conducting polymers. The conducting polymer is formed from a mixture of monomer and oxidant solution. The oxidant solution has a high concentration, and also includes a five or six-member ring compound with a functional group which acts as a retardant for the polymerization. Thus, the mixture of a monomer and oxidant solution exhibits excellent stability at room temperature. The conducting polymer accumulating in the space of the capacitor element can be more efficiently formed by using this high concentration oxidant solution. Therefore, the immersion and polymerization processes to form conducting polymer as the electrolyte of a solid electrolytic capacitor can be limited to only a few occurrences.