Last summer, Hanako Okada, a Tokyo lawyer and mother of two young children, started to plan a campaign for Parliament from the northern rural district where she spent her childhood. Nearly everyone she consulted gave the same odds on her chances of winning: close to zero.

As a candidate for a party in opposition, she was facing an incumbent from the party that has ruled Japan for all but four years since 1955. His grandfather, father and brother had all held seats in the prefecture before him. Ms. Okada, 44, was a political novice and a virtual stranger to locals in Hirosaki, the city in Aomori Prefecture on the northern coast of Japan’s main island that she had left more than a quarter century earlier to attend college.

Among democratic nations, Japan has one of the most abysmal records of giving women political power: Before a general election late last month, women held just over 10 percent of seats in the lower house of Parliament, putting the nation at 163rd out of 183 countries in the proportion of women in its national legislature, according to the International Parliamentary Union, a Swiss-based organization.

“I think everyone was thinking somewhere in their hearts that it would be impossible,” Ms. Okada said during an interview last week in a conference room at the telecommunications business in Hirosaki owned by her mother.

But in the general election, Ms. Okada, a candidate from the Constitutional Democratic Party, the largest opposition group, ousted the incumbent from the governing Liberal Democratic Party, Jiro Kimura, whose family who had not lost the lower house seat in that district for almost 40 years. Ms. Okada became the first woman to win in a single-seat district in Aomori, a rapidly aging, depopulating prefecture that produces 60 percent of the apples grown in Japan.