A stochastic theory of self-organization process of coherent macroscopic motion from system of Brownian particles is presented, starting with a set of coupled Langevin equations for the particles in potential that is not able to yield the macroscopic motion alone without interactions among the particles. A non-linear Fokker-Planck equation describing the self-organization processes is derived by utilizing the averaged field approximation. The Fokker-Planck equation is then discussed by using linear approximation. Instead of solving the Fokker-Planck equation except in a simple example, mechanochemical conversion is conveniently discussed by means of phenomenological equation in the velocity space by deriving the equation of motion from the Langevin equation. Hill's empirical equation that describes the dynamics of muscle contraction is easily obtained.