Means for easily determining the grade of malignancy of cancer cells. The amount of ATBF1 in the whole cell structure of test cancer cells separated from a living organism is detected, and on the basis of detection results, the grade of malignancy of test cancer cells is judged. Alternatively, the amount of ATBF1 in the nuclei of test cancer cells separated from a living organism is detected, and on the basis of detection results, the grade of malignancy of test cancer cells is judged. Still alternatively, the amount of ATBF1 in the cytoplasms of test cancer cells separated from a living organism is detected, and on the basis of detection results, the grade of malignancy of test cancer cells is judged. In a preferred from, at least one of (1) the amount of intranuclear presence and/or intracytoplasmic presence a region corresponding to exon 10 of an ATBF1 gene, (2) the amount of intranuclear presence and/or intracytoplasmic presence a region corresponding to exon 11 of an ATBF1 gene, and (3) the amount of intranuclear presence and/or intracytoplasmic presence a region corresponding to exon 3 of an ATBF1 gene is detected as the amount of ATBF1.