New census data has found that Japan's greying population of around 128 million grew by just 0.2 percent over the past five years, the slowest rate of growth since records began in 1920.
The number of people in Japan stood at 128.06 million last October 1, according to the government, which expects the population to roughly halve on current trends to 60 million by 2100.
The population has been declining year-on-year since 2007, and Japan last year lost the mantle of world's second-biggest economy to a rising China.
"It will be no surprise if we see the population decline for the first time in the next census, which will be carried out in five years," the official said.
Many young people have put off starting families, seeing it as a burden on their lifestyles and careers. A slow economy and tough employment conditions have also discouraged young people from having babies.
Japan's low fertility rate stayed flat in 2009, halting three years of modest increases. Japanese women have an average of just 1.37 babies in their lives, official data show.