The release of about 9.24 million kilolitres (58.1 million barrels) of reserves, announced on Monday, follows 1.26 million kilolitres from mandatory stockpiles, or three days' worth of demand, last week.
That was only the second release of reserves since a 70-day inventory cover system was introduced in 1993. In the aftermath of Hurricane Katrina in 2005, Japan left a lower reserve requirement in place for about four months.
With the latest measure, the ministry has relaxed the requirement to let companies hold 45 days' worth of oil.
The trade ministry said that since Monday, oil product shipments to metropolitan areas in the Kanto region around Tokyo have recovered to year-earlier levels, with 50,000 kilolitres transferred from west Japan, the draw down of stocks in the region and restarts at some refiners.
"If consumers don't hoard these products, the supply/demand balance in the Kanto region will be normalised," the ministry said.
The world's third-largest oil consumer had national reserves of 113 days of oil demand in government storage and 85 days held by the private sector at the end of December.
Some progress has been made in restoring energy supplies to the quake-stricken areas.
JX Nippon Oil & Energy Corp, a refining unit of JX Holdings , resumed operations at its Negishi refinery in Yokohama, south of Tokyo, on Monday. The refinery, which produces 270,000 barrels per day, has started production of gasoline and light oil to be shipped to areas damaged by the earthquake and tsunami.
Another major oil producer, Idemitsu Kosan Co , also said it began unloading oil products totalling 2,010 kilolitres at its Shiogama oil terminal in Miyagi Prefecture in the vicinity of the quake on Monday. The products included 1,050 kilolitres of gasoline, 470 kilolitres of kerosene and 490 kilolitres of gas oil.