Percentage of population by age in Japan.
0-14 years: 14.2%
15-64 years: 65.7%
65 years and over: 20%
Birth rate: 9.37 births/1,000 population
Death Rate:9.16 deaths/1,000 population
As you can see their death rate is only slightly lower then their birth rate. When you add in that the number of those over 65 is vastly greater then those under 15 you have a population in decline.
If you do not have people entering the work force at the same rate as you have people leaving it then you have a problem.
Greater efficiency will help, for a time, with the work situation but they need to figure out what social pressures are causing the decline.
They are and that is not the wonderful thing. The wonderful thing is that when they were at 90 million they were worried about overpopulation, and now they are way over 100 million, supposedly way into overpopulation, and worried about depopulation. Is that not amazing?