A method for accurately determining the particle diameter of gold colloid in a simple manner, and a gold colloid solution with a particle diameter within a desired range are described. A method for estimating the particle diameter distribution width of gold colloid based on the measurement of, in addition to the maximum absorption wavelength, a ratio (AλX/Aλmax) of the absorbance (AλX) at a wavelength differing from the maximum absorption wavelength to the absorbance (Aλmax) at the maximum absorption wavelength of the gold colloid solution, and the gold colloid solution obtained by the method are provided.