Provided is a novel method for treating cancer using an HSP90 inhibitor which exhibits a markedly superior antitumor effect and has a reduced side effect. An antitumor agent is characterized in that an azabicyclo compound of the following Formula (1) or a salt thereof is administered in combination with other antitumor agent(s).本發明提供一種使用表現出顯著優異之抗腫瘤效果且副作用較小之HSP90抑制劑的新穎之癌治療方法。本發明之抗腫瘤劑之特徵在於:併用投予下述通式(1)所表示之氮雜雙環化合物或其鹽與其他抗腫瘤劑。