The aim of this article is to propose a strategy to detect factors of a sound change by interpreting written texts. In the area of research in Japanese sound changes, written documents have been mainly used in order to inquire what sound changes occurred and when they occurred. A large number of scholars are liable to conceive that there are few issues that should be examined by written texts in this field. However, the scrutiny of well-known documents provides us with valuable sources that enable us to develop a new approach to dynamic aspects of a sound change. In this article, we deal mainly with Ken-syuku-ryou-ko syuu and Wazi syouran syou We discuss issues of the denasalizing process of prenasalized consonants in the 17th century. It is worth while to note that a phenomenon concerning this denasalizing was perceived in different manners by different documents. Some scholars may be apt to ask which document is better for us to access accurate information. However, such question is not adequate to our study. Interestingly, each document reflects each different aspect of the same linguistic phenomenon. This fact enables us to analyze aspects of the denasalizing process from diversified viewpoints. For this reason, it is significant for us to investigate into background of each document. That investigation reveals some factors of the denasalizing process.