A method for quantifying the expression level of human WT1 mRNA conveniently, in a short period of time, and with high sensitivity is provided. The method can be used for diagnosing cancer, such as leukemia and solid cancer, or for determining when to perform bone marrow transplantation. The method is for quantifying the expression level of human WT1 mRNA by one-step RT-PCR and comprises simultaneously subjecting the human WT1 mRNA and a housekeeping gene (mRNA) to reverse transcription and extension reactions carried out sequentially in the same vessel.