What is the fifth largest island of Japan?


In order to answer this question, we must consider at least three things:
  1. What is an island?
  2. What makes one island "larger" than another?
  3. What islands are part of Japan?

island?

An island is a piece of land completely surrounded by water.  What about a really large piece of land, such as Africa? or Australia? or Greenland?  Africa is normally said to be a "continent", not an "island".  Greenland is often said to be "the World's largest island".  Australia is sometimes said to be an "island continent".  What about small pieces of land, especially those which may disappear under the water at high tide?  What about land which is surrounded by water at high tide but which is part of a peninsula at low tide?  At what time of the day should we officially look for islands?

larger?

In English, a "larger" island is typically understood to be an island with greater area, for example, as measured in square kilometers.  However, translations of "larger" into other languages may sometimes result in ambiguity.  Having a "higher elevation" or having "more inhabitants" or having "more usable land" are possible alternative meanings to watch out for.  The context of discussion may even have an effect; for example, the "larger countries of Europe" typically refers to "the more populous countries", and Denmark is typically excluded from this group.  Even when an areal measurement is clearly given in square kilometers, the "method" of measurement may be important in critical cases.

Japanese territory?

The territory of Japan has changed over the years.  Some places, such as the islands of Okinawa, used to be outside of Japan.  Other places, such as the Pescadores, used to be part of Japan.  Today, Japan claims certain territories which are also claimed by other countries.  Some of the territory which Japan claims is occupied or visited by non-Japanese people.  Some of the territory which Japan claims is even controlled by a non-Japanese government, such as Russia.

the fifth largest island

Ranking the Japanese islands by area (square kilometers), including all of the islands which the Japanese government claims sovereignty over, we have:
  1. Honshu 本州 (227,949.70)
  2. Hokkaido 北海道 (77,982.31)
  3. Kyushu 九州 (36,733.48)
  4. Shikoku 四国 (18,298.14)
  5. Etorofu 択捉島 (3,182.65)
  6. Kunashiri 国後島 (1,498.56)
  7. Okinawa 沖縄島 (1,205.68)
  8. [Niigata 新潟] Sado 佐渡島 (854.17)
  9. [Kagoshima 鹿児島] Amami Oshima 大島 (712.32)
  10. [Nagasaki 長崎] Tsushima 対馬 (696.26)
  11. [Hyogo 兵庫] Awaji 淡路島 (592.13)
  12. [Kumamoto 熊本] Shimojima 下島 (573.97)

Etorofu would seem to be the fifth largest island. Excluding islands from consideration, for whatever reason, may change the ranking. The reasons for such exclusion are far more interesting than the actual ranking itself.