The fluctuations of credit ratings can make a considerable impact on a nation’s finance and economy. Moody’s 9-notch downgrading of the Korea’s credit rating from A1 to B- during the 1997 Asian currency crisis led to a rise in foreign currency loans’ cost, and this placed a heavy burden on the overall macro economy. When the credit rate is downgraded, the demand for that country’s bonds is generally decreased as the risk premium for them increases. This in turn causes costs for government debt to rise, thus presenting difficulties for the stable management of national finance. As a means to overcome the 2008 global financial crisis, major developed countries including the US have been implementing expansionary monetary and fiscal policies. Due to financial problems that emerged in the wake of these policies, a number of developed countries have had to witness the downgrading of their credit ratings. To take the US as an example, there was a large amount of friction in Congress in the first half of 2011 over the political debate on raising the debt ceiling. As a result, one of the world’s three major credit-rating agencies, Standard & Poor’s (S&P), downgraded the credit rating of US government bonds by a notch for the first time in the history of rating the country, stripping the US of its well-guarded AAA rating. [...]