The conditions of the applicability of a macroscopic description of the kinetics of micellization in a solution of a nonionic surfactant are revealed. Quasi-equilibrium concentrations of molecular aggregates in precritical and supercritical ranges of their sizes are determined. The kinetic equation of the formation of molecular aggregates in the near-critical and micellar ranges of their sizes is derived. Boundary conditions for the equation are formulated. Direct and reverse overcoming of the activation barrier of micellization by the aggregates is described. Quasi-stationary aggregates fluxes over the activation barrier of micellization are found.