TOKYO (AP) _ Continuing pessimism over the Middle East sent shares on the Tokyo Stock Exchange down further today, traders said. The dollar gained marginally against the Japanese yen.

The 225-issue Nikkei Stock Average lost 286.18 points, or 1.04 percent, closing the week at 27,329.55. It was the Nikkei's lowest closing level since Oct. 24, 1988, when it ended trading at 27,281.54.

A total of 320 million shares changed hands, down from the previous day's 380 million shares. On Thursday, the Nikkei plunged 893.41 points.

Traders said the stock market opened moderately higher, but market players then dumped issues because of pessimism over the situation in the Middle East.

Akira Shimoda, an equity analyst with Yamaichi Securities, said there had no news about major new developments in the Middle East today, but ''the situation there is hazy, resulting in a negative psychological impact on investors.''

The Nikkei index gained nearly 300 points in the morning, but later suffered from sporadic arbitrage selling to take advantage of price gaps between the spot and futures markets, he said.

Among losers were chemical, drug, shipbuilding and trading company issues.

The Nikkei index has lost a total of 4,634.19 points since July 20 because of rising interest rates in Japan and the Persian Gulf crisis, which began last Thursday when Iraq invaded Kuwait.

In currency dealings, the dollar finished the week at 149.75 yen, up 0.07 yen from Thursday's close of 149.68 yen. It opened at 149.75 yen and ranged between 149.53 yen and 149.85 yen.

''It's very quiet with only a small number of calls and customers,'' said Megumi Morita, a dealer with Sanwa Bank.

Ms. Morita said many market players conducted position adjustments in the absence of fresh incentives, including fresh news from the Middle East.

Meanwhile, oil prices remained steady in Tokyo throughout the day, said a trader with a major Japanese trading company.

North Sea Brent crude oil for October delivery was trading at $23.70 a barrel, unchanged from Thursday's close.

In bond dealings, the yield of benchmark No. 119 10-year Japanese government bonds closed at 7.995 percent, up from Thursday's 7.985 percent. Their price slipped to 83.47 points from 83.52 points.