A method of detecting a poor-prognosis stage I lung adenocarcinoma in a mammal, comprising a step of measuring the copy number of ACTN4 gene in the cells of the stage I lung adenocarcinoma tissue obtained from the mammal, wherein, when the copy number of the gene exceeds 4, the stage I lung adenocarcinoma in the mammal is predicted to be a poor-prognosis stage I lung adenocarcinoma.