TOKYO - Moody's Investors Service downgraded its rating of Japan's government debt by one notch to Aa3 on Wednesday, blaming a build-up of debt since the 2009 global recession and revolving-door political leadership, which has hampered effective economic strategies.

Japan is preparing to elect its sixth leader in five years to replace unpopular Prime Minister Naoto Kan, who is under fire for his handling of the response to the March tsunami and the subsequent radiation crisis at a crippled nuclear power plant.

The downgrade, while not unexpected, served as another reminder of the debt burdens that nearly all of the world's major advanced economies shoulder, even as policymakers struggle to agree on ways to stimulate sub-par growth without massive new spending.