A method is provided for determining a concentration of a target nucleic acid by using a nucleic acid probe labeled with a fluorescent dye. The method comprises: providing, as the probe, a nucleic acid probe capable of reducing fluorescence emission from the fluorescent dye when hybridized with the target nucleic acid; hybridizing the probe to the target nucleic acid; and measuring a decrease in fluorescence emission from the fluorescent dye after the hybridization relative to fluorescence emission from the fluorescent dye before the hybridization.