Japan's Population Density

Japan, which is roughly the size of California has more than one third the population of the United States (approximately 130 million). This makes Japan one of the most densely populated island countries in the world.

 The Japanese archipelago itself is 80% mountainous which forces most of Japan's population to inhabit the coastline and the two major plain regions of Japan: the Kanto （関東） and Kansai （関西） regions. Comparatively to the United States whose population density is 84 people per square mile, Japan's is nearly 10 times that amount at 836 people per square mile. Remember, that is with 80% of the island being mostly uninhabited. (Below is a link to convey the urban population density of Tokyo which is #1 in the world).

Not only does the limited habitable space force people to live near the coast, but being an island country, Japan relies heavily on the sea to provide food and materials for its people and it has been this way since Japan's first inhabitants arrived.

Links:

http://www.newgeography.com/content/002808-world-urban-areas-population-and-density-a-2012-update

http://www.factmonster.com/world/statistics/population-density-square-mile-countries.html