To decentralize Japan's fiscal system, the Trinity Reform, which was implemented from 2003 to 2007, reformed (i) the transference of tax revenue sources from the central to local governments,(ii) local allocation tax, and (iii) national subsidies and grants. This study drew on the multiple streams framework in public policy--including problem, policy and politics--to understand the financial change process in intergovernmental relationships and the successes of Prime Minister Koizumi as a policy entrepreneur in breaking the iron triangle of the Liberal Democratic Party politicians, ministry bureaucrats, and local governments to administer the fiscal decentralization and local autonomy reform.