The vast utility's entire stock of 17 reactors are now idle, including three units that suffered a meltdown when the tsunami hit Fukushima, as Japan warily eyes a spike in electricity demand over the hot and humid summer.
Only one of Japan's 54 units -- in northernmost Hokkaido -- is still working, and that is scheduled to be shut down for maintenance work in May.
The No. 6 unit at Tokyo Electric Power Co. (TEPCO)'s Kashiwazaki-Kariwa plant "stopped generating electricity at 23:59 Sunday, and its reactor was suspended at 1:46 Monday," TEPCO spokesman Osamu Yokokura told AFP.
The No. 6 unit is expected to undergo checks for several months, "but it depends on the result of checks and if we find some defects it may take more time to fix them," Yokokura said.
Japan's formerly-trusted nuclear power industry lost public confidence when the tsunami of last March knocked out cooling systems at Fukushima, sending three reactors into meltdowns.
Radiation was spread over a wide area, forcing tens of thousands of people from their homes and rendering farmland useless in the world's worst nuclear accident for a quarter of a century.
Reactors idled for tests must get the consent of host communities before being re-started, something many of those living near nuclear power plants are now unwilling to give, leaving power companies no choice but to rely more heavily on fossil fuels.
Japan's minister of economy, trade and industry Yukio Edano has said the government will not introduce a summer cap on the use of electricity nor the rolling blackouts that were carried out last year after the nuclear accident.
"We are expected to secure a stable supply of electric power for the time being," TEPCO president Toshio Nishizawa said in a statement on Sunday.
"But we call on customers to continue cooperating in saving electricity."