A method for predicting a clinical effect on a subject in WT1 peptide immunotherapy comprising the following steps: a) contacting a sample from the subject who has been given a WT1 peptide vaccine with a WT1 antigen peptide, wherein the WT1 antigen peptide comprises the same sequence as that of the WT1 peptide vaccine or an amino acid sequence consisting of 5 or more contiguous amino acids derived from the amino acid sequence of the WT1 peptide vaccine, and the WT1 antigen peptide has a length of 5-30 amino acids; and b) detecting the binding of the sample to the WT1 antigen peptide, and thereby, measuring an antibody titer of anti-WT1 antigen peptide IgG present in the sample, wherein an increase in the anti-WT1 antigen peptide IgG antibody titer in the subject means the achievement of a favorable clinical effect, and wherein the clinical effect is an overall survival rate, or a progression-free survival rate, and wherein the subject is a patient with a WT1-associated disease, and the WT1-associated disease is a leukemia including chronic myeloid leukemia, hematopoietic organ tumor including myelodysplastic syndrome, multiple myeloma, and malignant lymphoma, or a solid cancer including esophageal cancer, stomach cancer, colorectal cancer, pancreatic cancer, lung cancer, breast cancer, germ cell cancer, liver cancer, biliary tract cancer, head and neck cancer, skin cancer, sarcoma, kidney cancer, bladder cancer, prostate cancer, testicular cancer, uterine cancer, cervical cancer, ovarian cancer, thyroid cancer, carcinoid, pneumoblastoma, hepatoblastoma, brain tumor and thymic carcinoma, and wherein the WT1 antigen peptide comprises the amino acid sequence of any of SEQ ID NOs:7-56, 58-60, 62, 63 and 65-71, and wherein the WT1 peptide vaccine administered to the subject consists of the amino acid sequence of any of SEQ ID NOs: 2-6. a) contacting a sample from the subject who has been given a WT1 peptide vaccine with a WT1 antigen peptide, wherein the WT1 antigen peptide comprises the same sequence as that of the WT1 peptide vaccine or an amino acid sequence consisting of 5 or more contiguous amino acids derived from the amino acid sequence of the WT1 peptide vaccine, and the WT1 antigen peptide has a length of 5-30 amino acids; and b) detecting the binding of the sample to the WT1 antigen peptide, and thereby, measuring an antibody titer of anti-WT1 antigen peptide IgG present in the sample, wherein an increase in the anti-WT1 antigen peptide IgG antibody titer in the subject means the achievement of a favorable clinical effect, and wherein the clinical effect is an overall survival rate, or a progression-free survival rate, and wherein the subject is a patient with a WT1-associated disease, and the WT1-associated disease is a leukemia including chronic myeloid leukemia, hematopoietic organ tumor including myelodysplastic syndrome, multiple myeloma, and malignant lymphoma, or a solid cancer including esophageal cancer, stomach cancer, colorectal cancer, pancreatic cancer, lung cancer, breast cancer, germ cell cancer, liver cancer, biliary tract cancer, head and neck cancer, skin cancer, sarcoma, kidney cancer, bladder cancer, prostate cancer, testicular cancer, uterine cancer, cervical cancer, ovarian cancer, thyroid cancer, carcinoid, pneumoblastoma, hepatoblastoma, brain tumor and thymic carcinoma, and wherein the WT1 antigen peptide comprises the amino acid sequence of any of SEQ ID NOs:7-56, 58-60, 62, 63 and 65-71, and wherein the WT1 peptide vaccine administered to the subject consists of the amino acid sequence of any of SEQ ID NOs: 2-6.