A problem-posing system is developed with four phases including posing problem, planning, solving problem, and looking back, in which the "solving problem" phase is implemented by game-scenarios. The system supports elementary students in the process of problem-posing, allowing them to fully engage in mathematical activities. In total, 92 fifth graders from four different classes were recruited. The experimental group used the problem-posing system, whereas the control group followed the traditional paper-based approach. The study investigates the effects of the problem-posing system on students' problem-posing ability, problem-solving ability, and flow experiences. The results revealed more flow experiences, and higher problem-solving and problem-posing abilities in the experimental group. (Contains 7 figures and 5 tables.)