It is desirable for web applications to be reusable, for example in case a vendor wants to deliver new valuable services by using and combining existing applications quickly. One of the well-known ways of reusing web applications, Mashup, has become popular in the web development community and applied to many open web sites, as we can just combine the data retrieved from Web pages or APIs of the web applications. But in the case of the Mashup of enterprise web applications, or Enterprise Mashup, it is quite difficult since these applications are often not intended to be reusable and have no Web APIs. That means we have to create the Web APIs for the applications, which takes much work. In this paper, we describe a light-weight Web API creation methodology in order to create Web APIs for enterprise web applications much more easily than we can by extending the source code. The created Web API can connect to the web application just as a client who has an account does. So we can create a Web API for Enterprise Mashup without modifying the application at all and we can call the Web API securely using the user account information of the application. We developed the implementation of our methodology and applied it to actual applications to evaluate its effectiveness.