Japan’s $1.5 trillion state pension fund is likely to cut back its purchases of

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domestic stocks and foreign bonds this year, removing a key source of support for the Nikkei but providing some relief for the sliding yen.

With a major portfolio rebalancing out of the way, the Government Pension Investment Fund — the world’s largest pension fund—is expected to buy fewer assets within Japan and abroad, and may even need to sell assets as pension payments rise.

One of the biggest risks is for the Japanese government bond market, which is about to see a big increase in bond issues to pay for fiscal spending, just as the pension fund’s buying is expected to fade.

The pension fund was seen as one of the main drivers behind recent capital outflows from Japan that surprised the currency market with their size and persistence, analysts and traders said.

The rebalancing of the fund’s portfolio between October and March was cited as an important market factor, helping the Nikkei 225-share index hold above a 26-year low hit in October and playing a role in the yen’s broad slide to six-month lows against the dollar.

‘‘It was huge that there was buying by public funds,’’ said Toru Tanaka, senior manager for Mitsubishi’s treasury and foreign exchange office in Tokyo.

‘‘That did a lot to set the stage for the yen’s fall to 99 to the dollar,’’ he said. ‘‘If you can no longer hope for that to appear, it will be a positive factor for the yen.’’ Such buying was aimed at rebalancing the portfolio to keep the pension fund’s asset holdings in line with its preset target ranges, analysts said. After global stock markets slumped, it needed to buy domestic and overseas equities as its relative exposure to those asset classes shrank.

The pension fund may have to dish out more money than it receives this year as more of Japan’s baby boomers retire and begin receiving pensions, potentially leading the fund sell assets to raise cash.

The fund had earmarked ¥9.5 trillion, or about $95 billion, for financial market investment in the business year that ended in March. But starting this month, it will no longer have as much funding as loan payments from public entities dry up.

The fund ‘‘is not expected to sell or buy actively in stocks and foreign securities during the new business year,’’ said Takahiro Tsuchiya, a strategist at Daiwa Institute of Research. The giant pension fund is estimated to have bought ¥2.2 trillion to ¥3.3 trillion in overseas equities between October and March.