An aqueous composition may include: (A) a fluoride ion supply source in an amount that gives a fluoride ion concentration of 0.05 to 30 mmol/L in the composition; (B) a cation supply source in an amount that gives a mole ratio of cations of 0.3 to 20 to the fluoride ions in the composition; and (C) 0.0001 to 10 mass % of one or more compounds selected from a C4-13 alkylphosphonic acid, a C4-13 alkylphosphonate ester, a C4-13 alkyl phosphate and a salt thereof, with respect to the total amount of the composition, wherein pH is in a range of from 2 to 6.