The number of swine flu cases in Japan has risen rapidly - with more than 120 cases now confirmed.
Nearly 2,000 schools and companies have closed, and the government has discouraged unnecessary travel to quell the spread of the disease.
Just four cases had been confirmed in Japan as of Friday - people who had returned from Canada.
The first case in someone who had not been abroad - a 17-year-old student in Kobe - was reported on Saturday.
Call for calm
Japanese media said the new wave in infections would make Japan the fourth-most infected country in the world, after Mexico, the United States and Canada.
None of those infected are thought to be in danger of their lives, officials say, but the government is taking no chances.
"We must be careful, but with quick treatment patients can recover," said Prime Minister Taro Aso. "We must respond calmly and appropriately."