Provided is a method of measuring a biodegradation rate of a non-natural organic compound in the presence of a biodegradation medium, characterized by including: measuring a concentration of a radioactive carbon isotope 14C in a biodegradation medium; and measuring a biodegradation rate of a non-natural organic compound from a difference between the concentration of 14C in the biodegradation medium and a concentration of 14C in modern carbon. Further, a metal such as iron in the medium can be used as an internal standard. According to the method, a biodegradation rate of a non-natural organic compound can be measured safely, simply, rapidly, and accurately without providing any special measures against radiation.