As a first step to resolve the assignments of an end user development environment, this paper proposes a co-developing model for the participation of users in web application development. First, the procedure for a simple web application is arranged from the view points of “screen” and “screen transition” so that an end user can easily take part in the development. These can be classified into two parts; one that an end user can define and the other that an end user cannot. Using the definition items in these two parts, an operational model for co-developing is proposed, and the procedures for co-development are shown. A collaboration model that can be developed by a user and a system engineer is proposed; then, based on the proposed model, a tool supporting co-developing is built.