We have developed a methodology for developing roadmaps towards low-carbon society in local government. A quantification tool called "Backcasting Tool" (BCT) was developed. BCT estimates implementation schedule of all policies and actions considering their relationship, financial constraints of the actors, and co-benefit of the policies. The methodology was applied in Shiga prefecture, Japan, and a roadmap which consists of more than 240 policies is estimated considering direct costs paid by public and private sectors. As a result, cumulative implementation cost was 7.3 trillion yen in which public sector bear 17%. Cumulative emission reduction was 101MtCO₂, and average emission reduction cost was 73 thousand yen/tCO₂.