Human societies face various unsustainability problems, often characterized as “wicked” in the sense that they have no single definitive formulation. Thus, the role of creativity or insight in solving such problems has attracted a lot of attention from scholars. Therefore, this study investigated how an emerging methodology, Future Design (and its unique intervention of asking problem solvers to take a future generation’s perspective), can facilitate insight problem solving (IPS) and the generation of sustainable solutions. In a municipality in Japan, nine officers from a bureau responsible for water supply management participated in a series of seven Future Design workshops. In two groups, these officers created visions of water supply management 30 years into the future, taking the perspective of a future generation working in the same municipality. On the basis of in-depth transcription analyses of these workshops, we obtained a hypothetical framework demonstrating that four factors mediate the influence of perspective taking on IPS: (a) Discounting the present generation’s cost, (b) contrasting the future with the present, (c) deconstructing hierarchy, and (d) intellectual joy. While the first three mediators (a, b, and c) were considered to be contributors to the problem reframing and IPS via constraint relaxation, the fourth (d) was considered to do so via positive interpretation. Further, the reason why taking a future generation’s perspective is likely to lead to sustainable solutions, useful for the future—rather than the present—generation, is also discussed.