PROBLEM TO BE SOLVED: To provide a technique for quantifying circulating tumor DNA (ctDNA) which is a somatic mutation with high sensitivity from a cancer patient's living body sample, and to provide diagnosis, prognosis diagnosis, a therapeutic agent, and a technique for monitoring of cancer based on it.SOLUTION: In a test sample of blood or stool of a cancer patient, the copy number of DNA fragments by a gene mutation is measured, and if the mutation is present in tumor tissue of the cancer patients but not in normal tissue of the patient, the copy number is an index of the tumor burden in the patient.SELECTED DRAWING: None