The present invention provides an antitumor agent for treating HER2-positive tumors resistant or refractory to other antitumor agents. In more detail, the present invention provides a therapeutic agent for HER2-positive tumors that uses a pyrazolo[3,4-d]pyrimidine compound or a salt thereof as an active ingredient that is resistant or refractory to other antitumor agents.本發明提供一種用以治療對其他抗腫瘤劑具耐性或難治性之HER2陽性腫瘤之抗腫瘤劑。更詳細而言,本發明提供一種以吡唑并[3,4-d]嘧啶化合物或其鹽為有效成分之對其他抗腫瘤劑具耐性或難治性之HER2陽性腫瘤之治療劑。