By Suryatapa Bhattacharya and Chieko Tsuneoka

TOKYO -- The Tokyo Stock Exchange resumed normal trading Friday after an unprecedented all-day shutdown Thursday that executives blamed on a malfunctioning computer part.

The market opened as usual at 9 a.m. Friday, with the Nikkei Stock Average rising slightly.

Exchange executives apologized for Thursday's glitch.

"I keenly feel my responsibility as someone overseeing markets," said Koichiro Miyahara, chief executive of the stock exchange, which is operated by Japan Exchange Group Inc. "We will conduct an exhaustive examination into the cause."

The exchange's chief information officer, Ryusuke Yokoyama, said the machines involved weren't connected to the outside and there was no evidence of hacking.

At 7:04 a.m. Thursday Tokyo time, a magnetic-disk device used to store trading information malfunctioned, and a switch-over to a backup device didn't work, the exchange said. That left it unable to transmit price information properly.

Outages of major exchanges occur periodically, despite efforts by operators to fortify their systems, as electronic-trading volumes have soared in recent years and cyber-threats have mounted.

In July 2015, a glitch forced the New York Stock Exchange to halt trading for nearly four hours, and in February 2019, U.S. futures-exchange operator CME Group Inc. halted trading for nearly three hours due to technical issues. In Canada, glitches led the Toronto Stock Exchange to end the trading day early in separate incidents in April 2018 and in February of this year.

In August last year, the London Stock Exchange suffered an outage, its worst in eight years, which delayed the market open by more than an hour and a half. At the time the LSE said a technical software issue had temporarily prevented trading in a range of securities. The LSE has suffered two other outages since it implemented a new trading system in 2011.

German exchange operator Deutsche Boerse AG also experienced outages caused by faulty third party software earlier this year, affecting trading in derivatives and stocks.

In September 2019, Hong Kong Exchanges and Clearing said a bug in third-party software had led it to suspend activity on its derivatives trading platform

In August, a cyberattack from overseas caused four days of shutdowns at New Zealand's stock market.

Officials said a quicker resumption of trading might have been possible at the Tokyo exchange by restarting the system, but they feared disruption if accumulated orders suddenly flooded the market. They said it was safer to call off the entire day of trading Thursday, cancel orders that had been waiting before the market opened and make a fresh start Friday.

The shutdown was an embarrassment for a financial capital that has been looking to play a bigger role in Asian markets as concerns grow about Hong Kong's future.

At a news conference, Mr. Miyahara, the exchange CEO, was asked about the slogan of "Never stop" that the exchange and technology provider Fujitsu Ltd. had promoted when introducing upgrades in 2015. In response, he repeated an apology for causing market participants trouble. A Fujitsu spokesman referred questions to the Tokyo Stock Exchange. Fujitsu shares were roughly flat in early Friday trading.

Thursday's problem marked the first time an entire day of stock trading was halted in Tokyo since the exchange moved to a fully computerized trading system in 1999.

It led to some unusual scenes. Displays at the exchange, which typically processes more than a billion shares a day, showed zero volume for all market segments. Newspaper stock charts consisted simply of a series of dashes in place of prices.

Market participants said they believed the Tokyo Stock Exchange wouldn't suffer lasting damage so long as it can prevent the shutdown from happening again.

"This is the market that was open after the 2011 earthquake," said Takeo Kamai, head of execution services at CLSA in Tokyo. "This sort of thing has never happened before, so I think most investors will give it the benefit of the doubt."

Anna Isaac and Alexander Osipovich contributed to this article.

Write to Suryatapa Bhattacharya at Suryatapa.Bhattacharya@wsj.com and Chieko Tsuneoka at chieko.Tsuneoka@dowjones.com

(END) Dow Jones Newswires

10-01-20 2132ET