Nishio was born in Ōno, Hokkaidō, now part of the city of Hokuto.
After graduating Kyoto University, he became a public servant in 1973.
He was appointed deputy mayor of Hakodate by its then-mayor Hiroshi Inoue (politician) [ja] in July 2003, but resigned himself in December 2006 due to a rift with the mayor over the construction of a retirement home.
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