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2009年東京都議会議員選挙では，民主党が大幅に議席を増やす一方で，自民・公明の与党は過半数の議席を獲得できなかった．そして，多くのメディアが，この選挙結果を“歴史的”と評した．しかし，都議選では，過去にも社会党や日本新党などが大きな勝利を収めた経験がある．そこで本稿では，選挙区レベルのデータ分析を通じて，2009年選挙における民主党勝利の構造を再検討した．その結果，定数の大きな選挙区における得票率に，過去とは異なる特徴が見いだされた．また別の分析では，過去から現在に至るまで，定数の大きな選挙区では所謂「M＋ 1 ルール」が適用できないことも明らかにした． When the Democratic Party of Japan (DPJ) won 54 seats in the 2009 Tokyo metropolitan assembly election and became the first party in the Tokyo metropolitan assembly, the previous ruling coalition of the Liberal Democratic Party (LDP) and the Clean Government Party (CGP) lost the majority. The major media said that these electoral results are "historic," but a few parties (e.g., Japan Socialist Party in 1989, Japan New Party in 1993) already had huge electoral wins in recent elections. In this study, analyzing district level data, we reviewed the structure of DPJ's vote share in the 2009 election. The result, comparing recent elections, demonstrated that the features of the DPJ's electoral win are in the large magnitude. Additional analysis reveals that the "M+1 rule" is difficult to apply to the large magnitude.