Several calculations of the temperature dependence of the spontaneous magnetization of ferromagnet which have been published are not in coincidence with each other. In this paper, it is calculated by applying the cluster development of a gas to the spin-deviation gas—the Bose-Einstein lattice gas equivalent to the Heisenberg model of ferromagnetism. The results same as Dyson has given are obtained without meeting with the problem of non-diagonality, kinematical interaction or non-Hermitic Hamiltonian.