A pretreatment kit and a pretreatment kit for saliva in identification and quantitative determination of Streptococci mutans by immunochromatography utilizing an antigen-antibody reaction, which can remove aggregation caused by mucin and chain formation of Streptococci mutans in saliva in a simple operation and can efficiently flow out a complex of a labeled antibody and Streptococci mutans from a porous membrane retaining the labeled antibody, contains (A) a 0.01 to 10 mol/L aqueous solution of sodium hydroxide, (B) a 0.01 to 3 mol/L aqueous solution of tartaric acid and/or citric acid, and (C) a nonionic surface active agent and/or an amphoteric surface active agent, in which the component (C) is mixed with the components (A) and/or (B), or is provided separately, and at least one substance selected from the particular metallic salts is contained in at least one of the components (A), (B) and (C) in an amount of 5 to 25% by weight.