By Ministry of Foreign Affairs of Japan – http://www.mofa.go.jp/s_sa/sea2/ph/page3e_000444.html, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=48716261

Japan may be the oldest continually hereditary monarchy in the world, but it has one of the smallest lines of succession.

The line of succession is limited to males only through the Imperial Household Law. Only direct-line males may succeed to the throne.

It is discussed in Chapter 1: Article 1 of the Imperial Household Law of 1947: “The Imperial Throne shall be succeeded to by a male offspring in the male line belonging to the Imperial Lineage.”

Article 2 details how the line of succession should look:

1. The Emperor’s eldest son
2. The Emperor’s eldest son’s eldest son
3. The eldest son of the Emperor’s other descendants
4. The Emperor’s second son and his descendants
5. The Emperor’s other descendants
6. The Emperor’s brothers and their descendants
7. The Emperor’s uncles and their descendants

The line of succession to follow Emperor Naruhito is as follows:

1. Crown Prince Akishino (b. 1965)
2. Prince Hisahito (b. 2006)
3. Prince Hitachi (b. 1935)

There have been debates on changing Imperial Household Law to allow for female succession, but as of yet, nothing has been agreed upon or changed.