Japan's falling birth rate and ageing population have fuelled concerns about future growth in the world's second-largest economy. Experts had previously said the population was likely to start shrinking in 2007.
Preliminary figures released by the Ministry of Health on Tuesday showed that the number of deaths totalled 568,671 in the first half of this year, outpacing the number of births, which stood at 537,637 during the same period.
"This is the first time that we've ever recorded this kind of drop in the population during the first half of the year," a Health Ministry official said.
But he was wary of making predictions for the rest of the year, adding that it was impossible to say whether the half-year figures mean that the overall population would shrink.
Japan has projected that its population will peak next year at 127.8 million before dropping. But it is difficult to say whether Japan has seen its population start to fall earlier than expected, because of seasonal differences, the official said.
"This is the first time that we've ever recorded this kind of drop in the population"
Health Ministry official
"More people die during winter. Japan saw more than the usual number of deaths in March because of a more severe than usual flu epidemic," the official said.
"We are also seeing an increasing number of births after May and this summer, as we usually do," he said.
The "natural population" decrease was blamed partly on a flu epidemic that caused 100,000 deaths a month during January-March, although there were also 2400 to 5800 fewer births each month during the first half of the year compared with the period a year earlier.
Fertility and immigration
Japan's total population stood at 126,869,397 as of 31 March, according to a government report published late last month.
Immigration has been a negligible factor as Japan's government has remained cautious of widely opening its doors to foreigners.
The ageing population has raised concerns about the sustainability of Japan's pension system, which the government is trying to reform by cutting benefits and raising the contribution rate for individuals.
Japan's fertility rate - the average number of children a woman bears in her lifetime - hit a post-war low of 1.288 in 2004. Demographers generally consider a level of 2.1 as the "replacement rate" needed to keep a population from declining.
The government has been trying to encourage Japanese to have more children, in part by making it easier for parents to take time off work to be with newborns, and building more day-care centres.
But the efforts have proved unsuccessful. Late marriages and a growing number of people choosing to stay single are often cited as reasons for the declining fertility rate, along with the high cost of education, crowded housing and long working hours.
The falling population has started to open debate on the sensitive issue of whether largely homogenous Japan should open its doors to large-scale immigration.