This paper reconstructs the anelastic approximation in such a manner that it can be applied to any kind of fluid. As a result, it turns out that the work done by the buoyancy force in the approximation corresponds to the conversion between kinetic and internal energy. Also, the conditions for the applicability of the approximation are clarified. It is demonstrated, in addition, that the Boussinesq approximation is not a limiting case of the anelastic approximation.