A theory which is essentially equivalent to Bloch-Bethe-Van Kranendonk's formalism is developed of the properties of an ideal ferromagnet with isotropic exchange coupling between nearest neighbour spins 1/2. Concerning the thermodynamic properties of the system at low temperatures far below the Curie point a complete treatment is obtained. In this temperature region the excitation of the system is naturally described in terms of spin waves which obey Fermi-Dirac statistics. Interactions between spin waves are discussed in the scheme of the second quantization, and their effects to the spontaneous magnetization are also discussed.