preparing a sample including mRNA expressed from the target gene isolated from the cell;

preparing a dilution series of an absolutely quantified standard having a sequence of the target gene;

preparing a sample including mRNA expressed from a sequence of internal standard gene other than the target gene;

preparing a dilution series of an absolutely quantified standard having the sequence of an internal standard gene;

for each of the dilution series of an absolutely quantified standard having a sequence of the target gene and the dilution series of an absolutely quantified standard having the sequence of an internal standard gene, obtaining an amplification curve showing a relationship between the number of cycles when the amplification curve reaches a constant rate of increase of signal intensity (Ct value) and an amount of amplified DNA from a result of real-time PCR of the dilution series of the absolutely quantified standard, and creating a calibration curve based on the amplification curve;

amplifying the mRNA expressed from the target gene in the sample to determine a Ct value of the mRNA in the sample;

calculating an expression level of the target gene based on the Ct value of the mRNA in the sample and the calibration curve; and

obtaining the expression level of the internal standard gene of each sample of which the expression level of the target gene has been obtained based on the Ct value of the mRNA of the internal standard gene in the sample and the calibration curve, thereby standardizing the expression level of the target gene in each sample with the expression level of the standard gene, to obtain the standardized expression level of the target gene in which a variation between the respective samples has been corrected.