The rainy season started Tuesday in a wide area covering Japan's western half, 12 days later than last year, the Meteorological Agency said.
The rainy season in the area stretching from southern Kyushu to the Tokai region in central Japan started three to 11 days later than on average.
It ends around July 13 to 20 in an average year.
Since the wet season began May 18 in Okinawa and on Amami Island in Kagoshima Prefecture, other parts of Japan weren't affected until Tuesday because a weaker than usual Pacific high-pressure system hadn't activated the normal rain front, the agency said.
The weather was cloudy Tuesday in southern and northern Kyushu, Shikoku, and the Chugoku, Kinki and Tokai regions, and is forecast to be mostly cloudy or rainy in the coming week.
Except for the Pacific side of eastern Japan, the archipelago was relatively dry in most areas in May, particularly in western Japan, where rainfall on the Pacific side was 41 percent of average and the lowest since comparable data become available in 1946.