1. A gene expression level analyzing method comprising the steps of: acquiring expression levels of a plurality of target genes in a target cell every measurement time; extracting a maximum expression level and a minimum expression level from the expression levels of the target genes every measurement time; calculating a correlation coefficient of a frequency distribution of the gene having the maximum expression level at each measurement time and a frequency distribution of the gene having the minimum expression level at each measurement time; and comparing the correlation coefficient with a threshold value of the correlation coefficient.