This paper describes a quality assurance approach for source code to assure source code quality within less effort than conventional method. This paper introduces Data Relation Tracking Method (DRTM) as a practical instance of Data-Oriented Program Analysis, a key factor of our new quality assurance approach. DRTM helps comprehension of the source code by extracting the internal logic of the source code in declarative notation. An example and evaluation of DRTM for the C language is also described. The example shows that DRTM can deal in data structures and inter-data relations of any control structure. The evaluation shows that DRTM can extract the internal logic of the source code uniquely and that the extracted internal logic is useful for quality assurance of the source code.