The microservice architecture is essential for agile development and deployment of the application components; however, designing microservices for a web application is not a straight-forward task. One of the best ways to design microservices is to decompose a monolithic prototype of an application into microservices on the basis of both the complexity in engineering and the component boundaries of the application in the early phase of development. We propose a visualization tool allowing developers to interactively design microservice applications on the basis of the characteristics of source codes and the behaviors of a monolithic prototype. This visualization tool first constructs a calling-context tree from profile data taken in a dry-run of the application. Next, it generates an initial microservice design while considering keyword features in the source codes or amount of function calls between components. Developers can interactively refine this design via this visual interface by taking four-choice actions to revise boundaries of microservices while considering expected communications between them. This interaction will have a significant impact on runtime performance. Case studies of two open-source benchmark applications demonstrate the proposed tool enables interactive design of microservices. The results of the demonstration show that compared to the official microservice designs of the applications, the proposed tool can effectively design microservice applications.