TOKYO — For almost two decades Japan’s economic fortunes have deteriorated, and little seemed to be done about it.

But in the last few months, the nation’s new prime minister, Shinzo Abe, has pushed policy makers and other officials to take bold steps to revive one of the world’s largest economies. Their handiwork was evident on Thursday when the Japanese yen hit 100 to the dollar for the first time in four years.

Normally a weakening exchange rate might be taken as a sign of decline. The yen has fallen nearly 14 percent against the dollar this year, and no currency has fallen more except the Venezuelan bolívar. But in Japan’s case, it is a sign that the policies put in place by Mr. Abe and Haruhiko Kuroda, chairman of the Bank of Japan, are starting to work. A weaker yen makes Japanese exports more competitive around the world.

The most immediate impact of the weaker yen has been the boost in profits of the major exporters. This week, the Toyota Motor Corporation reported net income in the last 12 months jumped threefold, and Sony produced an annual profit for the first time in five years. Both companies forecast further profit increases largely because of the weaker yen.