We construct a Japanese grammatical simplification corpus and established automatic simplification methods. We compare the conventional machine translation approach, our proposed method, and a hybrid method by automatic and manual evaluation. The results of the automatic evaluation show that the proposed method exhibits a lower score than the machine translation approach; however, the hybrid method garners the highest score. According to those results, the machine translation approach and proposed method present different sentences that can be simplified, while the hybrid version is effective in grammatical simplification.