Eight patients died less than four months after undergoing liver operations conducted by the same doctor using laparoscopes, a type of endoscope, at Gunma University Hospital, the state-run institution said on Friday.
The hospital looked into 92 patients who had laparoscopic operations to remove part of their livers between December 2010, when the procedures started, and June 2014.
The eight patients died between several weeks and 100 days after their operations, hospital officials said.
It is not clear whether the operations caused the deaths, which were likely due to infectious diseases or liver failure, they said.
The hospital expressed regret over the eight deaths — five men and three women, all in their 60s to 80s.
In late August, the hospital set up a team of investigators, including outside experts, to look into the matter.
Surgical teams, including the doctor, failed to consult with an ethics committee on whether laparoscopy was appropriate for the patient in the case of all eight fatalities.
The hospital said the investigative team also looked into why laparoscopes were used in the operations.
The hospital has started to relay the investigative team's probe results to the bereaved families.