During one of Japan's busiest holiday weeks, more than 360mm of rain fell in 24 hours in parts of the Chiba area adjoining Tokyo, inundating roads and railways, with soldiers sent to help with rescue efforts.
"This case was an extremely unusual situation," Chiba Governor Toshihito Kumagai told reporters on Friday.
"I have responded to many disasters in the past, but I have never experienced a case like this."
The eight dead included one person trapped in a submerged vehicle, public broadcaster NHK said, citing Chiba authorities and police.
At Narita, airport staff distributed sleeping bags, water and snacks to hundreds of passengers, many of whom had bedded down on Thursday after transport disruptions stranded about 7000 people, causing widespread chaos.
"I queued for a taxi, but the wait was four hours," said Minoru Yamasaki, a 48-year-old Chiba resident returning from holiday, who had booked a hotel only to be told it could not put him up.
With nowhere else to go, he returned to the airport.
An airport spokesperson said most flights were expected to operate normally on Friday.
More than 20,000 homes were without power in the early hours of Friday, after rain flooded a power substation, utility Tokyo Electric Power said, while gas supply to nearly 13,000 was temporarily cut.
In Chiba city, one of the worst-hit spots, hundreds of people spent the night under foil blankets at makeshift evacuation centres in government buildings.
Some major highways in Chiba were shut, forcing drivers to take alternative routes that clogged traffic, highway operator NEXCO East said.
Several rail services were also suspended on Friday, though some services from Narita to Tokyo resumed, easing congestion at the international transport hub.
Weather authorities said the 367mm of rain over 24 hours in Chiba exceeded the previous record of 309mm in 2013.
