We conducted a Future Design deliberation workshop on the theme of the “3rd Environmental Master Plan” of Suita City, Osaka Prefecture over four sessions in 2019, with the participation of both city residents and officials of the city government. To condition the deliberations of participants, we adopted the method of Imaginary Future Generations (IFGs) and analyzed its impact on their future vision of the city in 2050, policy options needed to shape that future, and changes in their perceptions. We also investigated how the adoption of IFGs affects the relationships between personal attributes and the changes in their perception. The results of variance analysis and multiple linear regression analysis based on data from the deliberations and questionnaire surveys of participants revealed the following: (1) the content and quality of the 2050 vision of society and policy options conceived from the perspective of the IFGs significantly differed from those conceived from the perspective of the current generations; (2) IFGs heightened certain perceptions, such as “a sense of crisis about the future” and “a shared recognition of goals that are desirable for society as a whole”; and (3) although the degree of “critical thinking”, as a disposition of individuals, influences the heightening of perceptions in decision-making from the perspective of the current generations, when IFGs is adopted, the degree of “critical thinking” seems to be no longer a factor in heightening these perceptions. These findings could be useful for designing mechanisms to facilitate sustainable decision-making that considers the interests of future generations.