In this paper, we report on an experiment to monitor the user attention in a serious game called Le Village aux Oiseaux (The Birds' Village). The main motivation of this study stems from the importance of dynamically adapting scenario and challenges to the skills of each player. The solution we developed is a real-time, non-invasive visual attention monitoring method, using an evaluation of the reaction time in an open and continuous space. Our results provide information that can be useful to manage automatically each game session. It can be of benet to game designers as well, allowing them to use this information to improve gameplay, manage the diculty of game environments, and optimize positioning of visual resources on screen.