By Sam Nussey

TOKYO (Reuters) -SoftBank Group Corp on Monday booked an investment loss of $23.1 billion at its Vision Fund unit in the April-June quarter after a slide in the value of its assets, including closely watched unlisted holdings, amid market turmoil.

SoftBank had booked a record loss at the Vision Fund unit in May as market volatility driven by rising interest rates and political instability hit the tech investor.

SoftBank founder and CEO Masayoshi Son, who will speak at an earnings briefing from 4:30 p.m. (0730 GMT), has pledged to tighten investing criteria and preserve cash to weather the downturn.

In the quarter ended in June, falling listed investments included robotics firm AutoStore Holdings Ltd and artificial intelligence firm SenseTime Group Inc.

SoftBank said it wrote down the value of unlisted assets across its two Vision Funds by 1.14 trillion yen ($8.43 billion). Analysts have said writedowns of these private assets were unlikely to reflect the extent of current market weakness.

The second Vision Fund's stakes in 269 firms were worth $37.2 billion at end-June, compared with an acquisition cost of $48.2 billion.

Plunging initial public offering volumes and market scepticism towards money-losing startups have squeezed an important source of capital for SoftBank, which hopes to list chip designer Arm following the collapse of a sale to Nvidia (NASDAQ:NVDA).

To raise cash, SoftBank has exited companies including ridehailer Uber Technologies (NYSE:UBER) and home-selling platform Opendoor (NASDAQ:OPEN) Technologies, for a total gain of $5.6 billion.

SoftBank sold Uber at an average share price of $41.47, compared to the Friday closing price of $32.01.

The sell-off has also hit hedge fund Tiger Global, which competes with Softbank (OTC:SFTBY)'s "unicorn hunter" CEO Son on deals and saw its flagship fund fall 50% in the first half of the year after it underestimated the impact of surging inflation on markets.

Berkshire Hathaway (NYSE:BRKa) booked a $44 billion quarterly loss on its investments and derivatives, with Chief Executive Warren Buffett urging investors to ignore the fluctuations.

SoftBank's sliding portfolio pushed it to a 3.16 trillion yen net loss in the latest quarter. That compared with profit of 761.5 billion yen in the same period a year earlier.

($1 = 135.2600 yen)