We derivate the Langevin and the Fokker-Planck equations for the radius of
O(3)-symmetric subcritical bubbles as a phenomenological model to treat
thermal fluctuation. The effect of thermal noise on subcritical bubbles is
examined. We find that the fluctuation-dissipation relation holds and that in
the high temperature phase the system settles down rapidly to the thermal
equilibrium state even if it was in a nonequilibrium state initially. We then
estimate the typical size of subcritical bubbles as well as the amplitude of
fluctuations on that scale. We also discuss their implication to the
electroweak phase transition.Comment: 13 pages, LaTex file, To appear in Progress of Theoretical Physic