One of the main problems of web-based learning is staying motivated at a sufficientlevel. Learning games offering challenges and entertainment may stimulate student motivation for learning and mitigate this problem. Web-based learning support systems combined with learninggames may efficiently promote learning by encouraging student participation in learning. This study introduces a treasure hunt model, which represents the idea of inquiry-based learning usingset theory. We demonstrate this via a prototype of a web-based learning support system called OTHI, which employs an online treasure hunt game as the learning game. We integrate the soundlearning strategies of inquiry-based learning with the Web and online game technologies in this system. We expect that our learning support system will motivate students, and furnish an inter-active student-centered learning environment.