Most of the victims were in their 70s as the hospital specializes in taking care of senior citizens with chronic health conditions.
Seven people are still receiving medical treatment but the number of dead is likely to rise as six are said to be in serious condition.
Firefighters say the blaze was brought under control within half-an-hour but many died or were affected from inhaling smoke and toxic gases.
Police have taken an 81-year-old male dementia patient into custody on charges of arson.
Officers say they have surveillance camera footage of the suspect, identified only by his last name Kim, going in and out of the room that was found to be the origin of the fire.
The room was being used to store hospital supplies and was said to have no inflammable materials.
The hospital, which had claimed.. an electrical short sparked the fire, is suspected of having only one nurse on the night shift for the ward taking care of some 80 elderly stroke and dementia patients who would have needed physical assistance to escape.
Because the lack of medical staff on duty likely contributed to the high death toll, authorities are looking to see if the hospital broke the law by not having enough staff on stand-by.
Laah Hyun-kyung, Arirang News.