Japan's big manufacturers reduced greenhouse gas emissions by nearly 15 percent annually on average over the past five years compared to 1990, according to a survey published in the Nikkei daily.
The projected annual average reduction of 14.9 percent in the five years to March this year compares to the level in the 1990 fiscal year, the business daily said.
It said efforts by the big firms helped the country as a whole achieve its pledge under the Kyoto Protocol on global warming, to cut emissions by six percent from the 1990 level as of calendar 2012.
Japan is believed to have comfortably achieved this binding commitment, although there has been no official announcement.
Average emissions from all sources between 2008 and 2010 were 10.9 percent lower, if carbon trades with developing countries and forest sinks are taken into consideration.
Because forests absorb more greenhouse gases than they produce, countries can gain rights to additional emissions by protecting forests and replanting trees.
The Nikkei survey was released days after the government of pro-business Prime Minister Shinzo Abe, who came to power a month ago, began reviewing the previous administration's policies.
Included in the review is a 2009 promise by then-Prime Minister Yukio Hatoyama to slash Japan's carbon emissions by 25 percent from 1990 levels by 2020, provided other major polluters like China and the US also made sharp reductions.
The goal was seen as extremely difficult to achieve even at the time of its announcement.
But it was made even harder because of the huge rise in fossil fuel use since the nuclear disaster at Fukushima put Tokyo's atomic energy programme on hold.
The continued shutdown of nuclear reactors "could hamper reduction efforts," said the Nikkei. Along with the Abe government and the business community, it advocates the restarting of nuclear plants despite public opposition.
The earthquake and tsunami of March 2011 sent reactors at the Fukushima nuclear plant into meltdown and generated widespread distrust of a technology which previously provided around a third of Japan's electricity.
Explore further: Japan ready to ditch target for emissions cut