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