He could end up being dismissed from his position in the company, AFP reported.
Kyodo has also reported that Kelly has been arrested.
As head of the Renault-Nissan-Mitsubishi alliance, Ghosn has created an industrial behemoth, its combined 470,000 employees selling 10.6 million vehicles previous year from 122 factories around the globe.
The Brazilian-born Ghosn also serves as the chairman of Mitsubishi Motors; chairman and CEO of Renault; and chairman and CEO of the Alliance, a carmaking unit consisting of Nissan, Renault, and Mitsubishi.
Saikawa apologized at a press conference, saying he felt great disappointment. "I want to minimize the bewilderment and the impact on the operation and our business partners".
Dismissal of both Ghosn and Kelly would be "an unusual situation" as they are two of the company's three representative directors.
Ghosn and Director Greg Kelly have been under investigation at Nissan for several months after a whistleblower complaint, and the board is set to meet Thursday to remove them both. "I have never tolerated any wavering from that principle, I will never accept any interference", he once said. "This is serious misconduct".
If proved to be true, the suspected wrongdoing - the falsification of the securities reports, personal use of company funds for investment, and misuse of company expenses - could demonstrate that Ghosn was treating the company like his own personal possession.
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The company said it passed its findings on to Japanese prosecutors.
In 2003 he was named by Fortune one of the 10 most powerful people in business outside the USA and was considered one of the most respected business leaders in the world. Reports indicate that 2017 was the first time in four years that the chairman's salary had been under $8.9 million (1 billion JPY). His compensation, high by Japan's moderate standards for executive pay, has been an issue over the years.
And in February the French state, which owns a 15 percent stake in Renault, forced Ghosn to accept a 30 percent pay cut from the 7.25 million euros ($8.3 million) he took home as Renault CEO previous year, calling the amount "excessive". Japanese markets had already closed when the scandal broke.
Ghosn's ouster at Nissan comes as larger shifts are happening at the Renault-Nissan-Mitsubishi alliance.
Ghosn is credited with helping engineer a remarkable turnaround at Nissan over the past two decades, resuscitating the Japanese automaker from near bankruptcy after he was sent in by Renault.
In recent years, however, there has been discord in the company with regard to Ghosn.
"It's easy for a board to be alert when times are tough, but it's very important for a board to maintain their vigilance when times are good", she continues, adding that "sometimes boards can be deferential to an iconic CEO" such as Ghosn.