Using handheld devices in educational environments has begun to be recognized as suitable. Due to the drastic revisions in Japanese curriculum standards for elementary school children, new theories, systems and practices to support children in their learning are required. Therefore, we focused attention on science museums, which are different learning environments from schools. In this paper, we propose a system called Musex, which is used to support children in learning and exploring collaboratively in a museum with two PDAs (Personal Digital Assistants). Through Musex, children can collaboratively challenge some questions which are related to exhibitions. Solving the questions let children pay their attention to exhibitions naturally and understand contents of exhibitions more. We evaluated the system in a Japanese science museum and confirmed its effectiveness.