New Delhi (CNN Business) Carlos Ghosn's wife has slammed the Japanese legal system as "draconian" in a letter that says her husband has been "callously and needlessly" detained.

The nine-page letter from Carole Ghosn to watchdog group Human Rights Watch says that the former Nissan chairman has been held under "harsh physical conditions" since his arrest nearly two months ago on suspicion of financial crimes.

"No one should be forced to endure what my husband faces every day," Carole Ghosn writes in the letter, which urges Human Rights Watch to press the Japanese government for criminal justice reforms.

Carole Ghosn claims in the letter that her husband is being held in an unheated cell, where the lights remain on around the clock. She says he is allowed to bathe only two to three times per week.

The car industry legend has been in jail in Tokyo since he was first arrested on November 19. His son recently expressed concern about his health, saying his father had lost 22 pounds in detention.

Read More