Image copyrightGetty ImagesImage caption Carlos and Carole Ghosn in 2016 The wife of former Nissan boss Carlos Ghosn has said he is suffering under "harsh treatment" in a Japanese jail, where he has been detained for nearly two months.
In a letter to Human Rights Watch, Carole Ghosn described constant interrogations and appealed for action.
Mr Ghosn's arrest for financial misconduct shocked the auto industry.
His detention, which is likely to continue for months, has drawn criticism of Japan's justice system.
In Japan, suspects can be held while prosecutors investigate allegations.
They can also be detained for long periods once charges have been filed.Mr Ghosn, a towering figure of the car industry, faces three charges of financial misconduct including understating his income and aggravated breach of trust.
He has denied any wrongdoing.
"For hours each day, the prosecutors interrogate him, browbeat him, lecture him and berate him, outside the presence of his attorneys, in an effort to extract a confession," Mrs Ghosn said in her nine-page letter to the advocacy group.
There has been no immediate reaction to the claims from Japanese prosecutors.
Last week a judge said incarcerating Mr Ghosn was justified to prevent possible evidence-tampering and because of the risk that he might flee.
His defence team previously denied that the executive had been pressured to sign documents or a confession in Japanese, AFP reports.
In the letter, which has been widely reported, Mrs Ghosn describes the conditions of her husband's detention.
She said he is being held in a small, unheated cell and denied daily medications.
He has lost weight since his detainment, she said, and eats mainly rice and barley.Last week, the 64-year-old looked visibly thinner when he appeared in court for the first time since his arrest on 19 November.
"I urge Human Rights Watch to highlight his case to press the government to reform its draconian system of pretrial detention and interrogation," the letter said.
Image copyrightGetty ImagesImage caption Mr Ghosn has been charged with aggravated breach of trust and understating his income Mr Ghosn's lawyers said he could be in jail for another six months before his first trial is held.
"No one should be forced to endure what my husband faces every day, particularly in a developed nation like Japan, the third largest economy in the world," the letter said.
Brazilian-born auto executive was the architect of the Renault-Nissan alliance, and brought Mitsubishi on board in 2016.
In the past, he has been hailed a hero in Japan for turning around the ailing Nissan.