It's ridiculous to discuss this problem as if it was somehow unique to Japan. As nurizeko said, most developed nations have a low fertility rate. Here is some hard data from the US Census Bureau's International Data Base (no link, google for it):

TFR (TOTAL FERTILITY RATE) IN 2005
Belgium: 1.6
France: 1.8
Germany: 1.4
Italy: 1.3
Japan: 1.4

By the way, TFR is defined as the average number of babies born to women during their reproductive years, and a TFR of 2.1 is usually considered the replacement rate. Admittedly, the value for Japan is low but not exceptionally so by a European standard. Please also note that there is no reason for Belgians to be smug about their fertility rates: they're about average in Europe and far from the replacement rate.