A method and apparatus for measuring a molecular composition of protein molecules. A fluid containing at least two varieties of protein molecules is irradiated with first and second laser light beams having different wavelengths. The molecular composition of the at least two varieties of protein molecules contained in the fluid is measured on the basis of an autocorrelation function of a scattered light signal from the at least two varieties of protein molecules irradiated with the first and second laser light beams having different wavelengths.