Provided is an aqueous liquid composition comprising a WT1 protein-derived cancer antigen peptide, wherein the peptide is stabilized. An aqueous liquid composition comprising a peptide and an excipient, and having a pH of 3 - 6: wherein the peptide is selected from the group consisting of the following (A) and (B), (A) a peptide consisting of the amino acid sequence shown by Arg-Met-Phe-Pro-Asn-Ala-Pro-Tyr-Leu (SEQ ID NO: 1), and (B) a peptide consisting of the amino acid sequence shown by SEQ ID NO: 1, wherein 1 to 3 amino acids are deleted, substituted and/or added, and having a cytotoxic T cell-inducing ability; and the excipient contains one or more kinds selected from the group consisting of the following (C), (D) and (E), (C) one or more kinds of alpha hydroxy acids selected from the group consisting of glycolic acid, lactic acid, malic acid, tartaric acid, citric acid and pharmacologically acceptable salts thereof, (D) one or more kinds of dicarboxylic acids selected from the group consisting of malonic acid, succinic acid, glutaric acid, maleic acid and pharmacologically acceptable salts thereof, and (E) methionine.