Watanabe was born 5 March 1907 in the village of Uragawara, which now is Joetsu, in Niigata Prefecture, Japan. His parents were farmers. He studied at Niigata Prefectural Takada Agricultural High School before moving to Taiwan where he lived and worked for the sugarcane company Dai-Nippon Sugar Company though until the end of World War II when he returned to Joetsu. He lived with his son's family until 2015 when he aged 108 got infected by influenza and moved to a retirement home. He has in total 5 children, 12 grandchildren, 16 great-grandchildren and one great-great-grandchild, who was born on 15 January 2019, just five days before he became the oldest living Japanese man after 113-year-old Hokkaido man Masazō Nonaka's death.