Method for measuring a depth of a defect inside an inspection object is provided. The method comprises steps of: generating thermal image data corresponding to a temperature of a surface of the inspection object by photographing a heated surface of the inspection object at a predetermined time interval by a photographing device; obtaining a temperature curve showing a temporal change in temperature of the surface of the inspection object based on the thermal image data; fitting a theoretical equation obtained from a heat conduction equation including a parameter related to the depth of the defect of the inspection object to the temperature curve to obtain a theoretical curve showing a temporal change in temperature of the surface of the inspection object; and obtaining the depth of the defect of the inspection object based on a value of the parameter in the theoretical equation corresponding to the theoretical curve.