This paper is one of works in which I intend to clarify the town planning methods of Japanese castle towns. In this paper I study on the process of constructing covered water works in castle towns. The summary is as follows : The water works in castle towns were recognized as an important town facilities. The water works with covered ducts began to be constructed from about sixteen twenties after open water works were developed, and served mainly from the mid of seventeeth century. The adopting of water works with covered duct is considered to be caused by the change in quality of castle towns from being military points at its first stage to becoming cities where many people gathered to live in the mid of seventeenth century.