The Meteorological Agency said Monday that a tropical storm was off the southern coast of Japan's southwestern main island of Shikoku. It was heading northeast, packing winds of 108 kilometers (66 miles) per hour.
The agency said the storm could dump up to 300 millimeters (1 foot) of rain in central Japan by Tuesday evening.
Another storm is expected to hit near southern Japan later this week.
Officials say Typhoon Hagibis, which hit Japan earlier this month, left at least 70 people dead and 12 others missing. Japan's Kyodo News agency, citing its own tally, put the death toll at 81.