Nowadays, commercial CAD packages with knowledge base systems which support creative design activities are released in succession. However, it is a new burden for the designer to input knowledge to the knowledge bases. In this paper, we propose our idea to generate design rules from design documents written in natural language that are accumulated in enterprises to manage design knowledge, and we also propose a design rule generator implemented based-on our idea. This design rule generator not only reduces the burden to input the rule codes to knowledge-base CAD, but also facilitates design knowledge management, because it is easier for the designer to compose and manage the documents than the rule codes.