The idea of Japan joining the EU, why not.

The followings are considered to be the conditions to join the European Union (EU) or any other possible economic and political union of countries/states. 

- Microeconomics:
  1. The high cross border trade (export and import) frequency
  2. The labour and capital mobility across the border

- Macroeconomics:
  3. The synchronised business cycle (boom and recession) among the member states
  4. The balanced fiscal policy neither disrupting the fiscal and monetary policy conducts nor depreciating the economic credibility of the entire union.

- The other aspects:
  5. Cultural and historic tie among the member states
  6. Geopolitical diplomatic interest

Japan fulfils these conditions except for the forth one because of the massive national debt of Japan. The second and third conditions might be questionable but they are not insignificant at all.

Remember that the foreign tourism is accounted as export&import!  The EU members and Japan trade not only physical items but also tourists a lot with each other. 

Also remember that Japan had had the international trade with Europeans more than the others since 17th century till the end of Shogunate era! In particular, during Tokugawa dynasty, lasting for approximately 300 years, the Netherlands was only the country which was permitted to trade with Japan implementing the national isolation. Before then, Spanish and Portuguese were the major diplomatic partners until they were banned from it. 

Both the EU and Japan very often share the remarkable foreign diplomatic and geopolitical interest. For example, both frequently suffer from Russian aggressions and the geopolitical interests of containing Russia. Both the EU and Japan are required to maintain their sea-lane for both economic and political stabilities so that they had better share their sea-lane and its security.

Thus, I have actually posted on a social network page of European Federalist to request Japan to join the EU to fill the membership position of the United Kingdom of Great Britain and Northern Ireland (UK). 

PS. This is just a simulation which is not feasible in real so that please do not take this issue seriously.