  We introduce a mean-field type approximation for description of company's
income statistics. Utilizing huge company data we show that a discrete version
of Langevin equation with additive and multiplicative noises can appropriately
describe the time evolution of a company's income fluctuation in statistical
sense. The Zipf's law of income distribution is shown to be hold in a
steady-sate widely, and country-dependence of income distribution can also be
nicely implemented in our numerical simulation.
