TOKYO — Japan is opting for another round of shock treatment, in a stark admission that the country’s economic revival plan is faltering.

After insisting for more than a year that its aggressive monetary action was sufficient, the Bank of Japan on Friday unexpectedly announced that it would buy larger quantities of government debt. By injecting more money into the economy, the central bank is trying keep borrowing costs low, encourage spending and, ultimately, stoke inflation and growth.

The bold move helped push stocks higher around the world. The Standard & Poor’s 500-stock index rose 1.1 percent on Friday, and European equities ended the day up more than 2.5 percent.

In Japan, the Nikkei 225-stock index average hit fresh highs, jumping almost 5 percent for the day. The yen fell to its weakest level against the dollar in a month.