The purpose of the present paper is to review investigations of the antiferromagnetic Heisenberg model on the kagomé lattice and to propose some schemes based on the spin-wave theory by which physical properties of the present model can be better understood. By means of our schemes, it is shown that the linear-spin-wave flat mode of the present model comes from the semi-local symmetry of the linear-spin-wave Hamiltonian, where the flat mode has an excitation-spectrum with ω≡0 for all k. It is also shown that the inclusion of higher-order quantum fluctuations may break the symmetry and thereby the flat mode may be lifted up.