Provided is a polymerization solution for electrolytic polymerization containing a borodisalicylic acid and/or a salt thereof as a supporting electrolyte. The polymerization solution inhibits precipitation due to the hydrolysis of borodisalicylic acid ions, and provides a conductive polymer exhibiting excellent heat resistance. The polymerization solution according to the present invention has: a solvent consisting of 100 to 80 mass% water and 0 to 20 mass% organic solvent; at least one type of monomer having a [pi]-conjugated double bond; at least one supporting electrolyte selected from the group consisting of borodisalicylic acid and borodisalicylates; and at least one stabilizer selected from the group consisting of nitrobenzene and nitrobenzene derivatives. The polymerization solution is characterized by the stabilizer content being more than 1/8 mol per 1 mol of supporting electrolyte. A complex is formed by the stabilizer and the borodisalicylic acid, and the formation of precipitates due to the hydrolysis of borodisalicylic acid ions is inhibited.