Authorities are pursuing three separate lines of enquiry against the 64-year-old Franco-Lebanese-Brazilian executive, involving alleged financial wrongdoing during his tenure as Nissan chief.
They suspect he conspired with his right-hand man, US executive Greg Kelly, to hide around half of his income of five billion yen (around 30 million euros) over five fiscal years from 2010.
They also allege he under-reported his salary to the tune of four billion yen over the next three fiscal years, apparently to avoid criticism that his pay was too high.
The extension that prosecutors won on Monday allows them to continue investigating a third claim that alleges Ghosn sought to shift a personal investment loss onto Nissan's books.
As part of that scheme, he is also accused of having used Nissan funds to repay a Saudi acquaintance who put up collateral money.
Prosecutors have pressed formal charges over the first allegation but are yet to do so for the others.
Ghosn, who denies any wrongdoing, will reportedly be allowed access to his lawyers on 2-3 January – both Japanese holidays.
Tensions among members of the car alliance
While Nissan and Mitsubishi Motors quickly sacked Ghosn from his leadership posts after his arrest, Renault has responded more cautiously to the allegations.
The French firm has also repeatedly called on Nissan to organise a shareholders meeting and is reportedly seeking increased representation on the board of the Japanese automaker.
Nissan has rejected the calls and says it is waiting until a commission looking into governance at the firm releases its findings.
The Japanese automaker has also so far failed to agree on who should succeed Ghosn as chairman.
We Are All Carlos Ghosn
In Lebanon, news of Ghosn’s dramatic fall has united the country’s fractious politicians around him. A large billboard was put up on 6 December in the streets leading to downtown Beirut which reads: “We are all Carlos Ghosn”.
Ghosn was born in Brazil, but also holds Lebanese citizenship.
Earlier in December, Hady Hachem, the chief of cabinet of the minister of Lebanon's foreign affairs, suggested that Ghosn's treatment by the Japanese legal system was excessive.
“Carlos Ghosn saved tens of thousands of jobs in France and Japan,” Hachem said in an interview. “If he made a mistake, he should be held accountable for it. But this should be done in a decent way. Why is he being humiliated in this manner?”
“The Lebanese state is acting only because it’s suspicious of the way Ghosn was arrested and treated as if he were a dangerous terrorist or a war criminal,” Hachem concluded.