A liquid aqueous composition comprising a cancer antigen peptide derived from WT1 protein and additive(s) and having a pH value of 3-6, wherein the peptide is stabilized. In the liquid aqueous composition: the peptide is selected from the group consisting of (A) and (B) (A) a peptide that comprises an amino acid sequence represented by Arg-Met-Phe-Pro-Asn-Ala-Pro-Tyr-Leu (SEQ ID NO:1), and (B) a peptide which has an amino acid sequence derived from the amino acid sequence represented by SEQ ID NO:1 by deletion, substitution and/or addition of 1 to 3 amino acids, and is capable of inducing cytotoxic T cells； and the additive(s) are one or more members selected from the group consisting of (C), (D) and (E) (C) glycolic acid, lactic acid, malic acid, tartaric aid, citric acid and pharmacologically acceptable salts thereof, (D) malonic acid, succinic acid, glutaric acid, maleic acid and pharmacologically acceptable salts thereof, and (E) methionine.
