A biological sample containing a target nucleic acid is mixed with a primer, a substrate labeled with fluorescence, and DNA polymerase to prepare a test solution. After a nucleic acid amplification reaction is performed in a predetermined manner, the test solution is attached dropwise on a slide glass, which is mounted on a sample holder of an inverted fluorescence microscope. Data measured by a photomultiplyer are arithmetically operated by applying an autocorrelation function in a data processing apparatus. Based on the arithmetic results, quantification data for the target nucleic acid, are numerically or graphically displayed on a screen of a display apparatus.