PURPOSE: To enable measurement of a fusion condition with high accuracy by bringing a soln. of ionophore of cation and a specific fluorescent material into contact with a membrane fusion matter and measuring the change in the intensity of fluorescence.

CONSTITUTION: A fusion matter obtd. by membrane fusion of the liposome obtd. from the coliform bacilli exposporium, etc. and contg. polin and the liposome contg. a K ion soln. without contg. polin is added to a soln. of alcohol, etc. of ionophore that can be bond selectively with specific cation, for example, valinomycine to K+ and a fluorescent material which migrates according to negative chemical potential and allows passage of liposome (phospholipid membrane) (cyanine dye, merocyanine dye, etc. of 3,3'-diisopropylthiodicarbocyanine iodide, etc.). Excitation wavelength is first irradiated thereto right after the addition and the intensity of the fluorescence emitted by such irradiation is measured. The change in the intensity of the fluorescence with time is plotted with time and the rate of membrane fusion is measured with good accuracy by making use of the phenomenon in which the degree of quenching of the fluorescence is lower as the fusion is more progressed.