The focus of this paper is on user participation in product design process. Ours is a human-centred approach in which users' expertise is acknowledged and which addresses the development of future products, rather than the current situation in users' work. This type of user participation applied to co-creation is possible when the users' prospective work is made visible. This paper presents a tool with which product users can experiment working with the product under design. As the product designers can observe the users' actions with the tool, their understanding of the users' work practice increases and their discussions with the users improve. The tool, VIP2M, is a virtual environment for prototyping a mobile working machine and constructing it in a walk-in virtual environment. We built it following the tradition of design science research and evaluated it by user tests. The study shows that virtual environment is a useful base for constructing appropriate tools for product users' participation in the product design.