ALONG with America, Japan was caught by surprise when European countries began queuing up in March to join China’s new Asian Infrastructure Investment Bank (AIIB). Officials from Japan’s finance ministry had advised Shinzo Abe, the prime minister, that Britain, for one, would never sign up (it was the first major Western country to put its hand up to join). Now that both Australia and South Korea are taking part, Japan is the last big holdout in the region. It is in a quandary...

Why they should join?
Since Japan would help shape the economic development of countries in the region, just as it does already through the Asian Development Bank (ADB), which was founded in 1966 with Japan and America as the biggest shareholders, why shouldn't they welcome it then?

Some in Japan argue that taking part would improve strained ties between the two countries...Supporters in Japan of their country’s membership say Japan could act as a counterbalance to China’s influence inside the bank.

Why hasn't they join?

Yet officials close to Mr Abe say he continues to view the AIIB with deep suspicion, as a means to expand China’s strategic and economic power. Since it would be almost impossible for America to join the AIIB against the will of Congress, with its many critics of China and abhorrence of funding any new international bank, Japan would be breaking ranks with its ally should it join. It would look especially odd so soon after a trip by Mr Abe to Washington in April that was aimed at strengthening Japan’s alliance with America.

http://www.economist.com/news/asia/21652351-will-japan-lend-its-muscle-chinas-new-asian-infrastructure-bank-join-or-not-join

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Hmm... let's see...

Japan-America VS Japan-China

This smells of conspiracies... How did China managed to convince European countries to join? If Japan join, does America has to join too???

This is gonna impact the economy world-wide! Maybe even more than IMF or World Bank...

The AIIB sounds like a good idea, though I am sceptical as to how pure it is. They refused to accept North Korea and Taiwan, I am worried about the latter, since that does not correspond to funding the developement in Asia.
Also, I don't get why there are so many different associations for internatioanl funding, I guess they're all dominated by some superpower? (like China)
I think they should join, to strengthen their asian relations, which are quite critical. Japan is already a big player in the ADB, so I think western relations should be fine.
Then again, I don't know much about economy and scrapped my knowledge from Wikipedia.

Dude they hate China, no way that would happen!

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I think Japan doesn't to join

bump

I agree with the answers you already gave. Personally though I think they should join. America might have a hissy fit but meh, they do that often anyway.

no way

no way never

Interesting fact:

AIIB is also another acronym for Japanese Red Army, the Anti-Imperialist International Brigade (AIIB)...

http://q.gs/8TIAR

Latest Update: Signing ceremony

Why is China doing this?

- Both diplomatic win & achieve their economic objective
- Diplomatic:

China is frustrated with their lack of influence in the big global financial institutions such as the World Bank and so AIIB has been created to increase their influence

- Economic Objective(Beijing master plan):

Instead of building their engineering giants at their home ground, they need somewhere else to build ports, roads and cities.

One way to do so is to lend the region the money to pay Chinese companies to build a 21st Century Asia with Chinese influence at its core.