The cells that make up the body continuously undergo cell division to maintain life, but stress and carcinogens, and physiological stimuli suppress immunological functions in the body and change cells to cancerous cells. The only way to overcome cancer is by treatment through early detection and proper diagnosis. However, because early stage of cancer shows subjective symptoms that are similar to those of other common illnesses, even doctors have difficulty separating cancer from common illnesses in its early stage. However, the Ki(bioenergy) present in our body recognizes and changes to the most minute changes in the body. This method of detecting the change in bioenergy to discover a disease in the early stage to raise treatment success is called the OBET: O-Ring, Bioenergy Test. This method, which is based on the theories of oriental medicine, focuses on tumors, especially malignant tumors, and has shown value in diagnosis and treatment, as witnessed in the clinical applications. The diagnosis method and cases of OBET: O-Ring, Bioenergy Test are presented.