It is well known that sentences in Japanese patents have long and complicated structures, especially necessary conditions and details. Here, patent sentences are analyzed and classified by pattern of modified relationships. Morphemes were first extracted using the famous morpheme analysis tool Chasen, and then the modified relations were extracted using the software Cabocha. Many modification mistakes were caused by long complicated structures, which required correction by humans. In the process of correction, the modification structure patterns were classified using about 200 sentences. This clarified the characteristics of Japanese patent sentences, and it is useful in machine translation of patent sentences.