Anyone invested in the Japanese stock market? Any recommendations or why do people not invest in Japan?

Very good question, after some decades of negative performance,

there should be some potential ?

As far as I know there is no dedicated Japan Technology index ETF ?

That would be a nice one: NASDAQ Japan Technology Index versus S&P500, not too bad over the last ca. 8 years.

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It depends on your goals I think - if you're expecting rocketing growth like the US markets, I don't think Japan's market would be the best option. However, if you plan to look into established companies with decent dividend payouts and government support (Mitsubishi, Sumitomo, etc are a few examples of companies that Berkshire Hathaway have recently invested in), then the Japanese market is worth taking a look at. Plus, you should take Japan's recent change in leadership into consideration as well!