The invention provides an aqueous liquid composition comprising a WT1 protein-derived cancer antigen peptide, wherein the peptide is stabilized. The aqueous liquid composition contains a peptide and an excipient, and has a pH of 3-6. The peptide has the amino acid sequence Arg-Met-Phe-Pro-Asn-Ala-Pro-Tyr-Leu (SEQ ID NO: 1), wherein optionally 1 to 3 amino acids are deleted, substituted and/or added, such that the peptide has a cytotoxic T cell-inducing ability. The excipient is (a) an alpha hydroxyl acid selected from glycolic acid, lactic acid, malic acid, tartaric acid, citric acid and pharmacologically acceptable salts thereof, (b) a dicarboxylic acid selected from malonic acid, succinic acid, glutaric acid, maleic acid and pharmacologically acceptable salts thereof, and/or (c) methionine.
