TOKYO — Japan’s long-sluggish economy has found its spark. To keep it flickering, Japan needs to find more big spenders like Sayaka Sakata.

Ms. Sakata, a 49-year-old mother of four, recently moved to a rented house from an apartment in the city of Yokohama after her husband got a new job. That meant buying new stuff: a washing machine, a refrigerator, some appliances and even a used car. Saving, she said, was no longer her top priority.

“I’m an optimistic person,” she said. “I think spending money on my family is more important.”

The unexpected surge in economic growth reported in Japan this week was thanks in large part to an unexpected source: Japanese consumers.

Long known as aging penny pinchers, Japanese shoppers in recent months have decided to spend. Data showed they bought more cars, air-conditioners, television sets and other appliances during the three months that ended in June, fueling an unusually high annualized growth rate of 4 percent.