Flocs formed of polystyrene latex particles undergoing rapid Brownian coagulation were counted directly under an optical microscope to verify the validity of our previously proposed equation to predict the temporal evolution of the concentration of flocs developing fractal structure. Experiments were carried out by changing the diameter of the primary particle in order to emphasize the difference of excluded volume of formed flocs. The obtained results demonstrated the enhancement of the rate of coagulation in the later stage of coagulation, which verifies the validity of our previously proposed equation.