Reliable real data is indispensable for the examination, evaluation and the improvement of the organizational structure. This paper proposes a method to use patent documents for analyzing organizational structure of researchers. The method is more efficient and objective compared to personal interview. The structure of research groups is modeled as a "inventors graph", which is a directed graph where each node represents an inventor and an edge represents co-inventor relationship. Empirical evaluation is conducted to cosmetic related companies and their patents that applied between 1998 and 2002 in Japan. It is shown that there is different characteristics in the inventors graph between Japanese companies and foreign companies. Moreover, time series analysis revealed that the inventors graphs of a Japanese company Kao changed in 2001 to foreign company type.