Kim Young-oh lost a daughter when a ferry capsized on April 16, killing at least 294 people, mostly students on a school excursion.
"Kim was so emaciated he could not even sit straight," said Yoo Kyung-geun, a spokesman for the victims' relatives. "He was on the verge of losing consciousness or even losing all his life functions."
Kim has been on hunger strike for 40 days to demand an independent probe into the disaster, Yonhap news agency said.
He was being treated in hospital in north-eastern Seoul.