Surgeons at Shinshu University Hospital in Matsumoto, Nagano Prefecture, started a transplant operation Tuesday in which former Foreign Minister Yohei Kono would receive part of his son's liver.
The operation on the 65-year-old Kono, whose hepatitis C condition and liver cirrhosis have deteriorated, was expected to last until early Wednesday.
Kono now heads a small faction in the Liberal Democratic Party.
His son Taro, 39, is a parliamentary secretary at the Public Management, Home Affairs, Posts and Telecommunications Ministry. Taro Kono underwent tests last week in relation to the procedure.
The surgical team overseeing the operation is headed by Seiji Kawasaki, the sources said.
According to Kono's office, the former foreign minister was admitted to a Tokyo hospital in late January after developing cirrhosis of the liver. For a time, he hoped to recover naturally.
In late February, however, doctors suggested that he undergo a liver transplant, according to informed sources.
Despite initial reluctance, Kono decided to undergo surgery out of a desire to continue his political career.
He was transferred to Shinshu University Hospital earlier this month, his office said.
Shinshu University Hospital is a pioneer in terms of liver transplants involving live donors.
Staff there have worked on 160 operations of this kind, and the hospital is authorized to perform liver transplants involving brain-dead donors.
More than 1,800 liver transplants involving live donors have been performed in Japan.