To investigate the spin wave and the stability of the ferromagnetic Kondo lattice, we describe the state with Sz = S -1 and that with Sz = S -1/2 in terms of the variational wave function. In this model, it is found that the spin-wave spectrum does not differ appreciably from the RPA result, since the bottom of the individual-particle excitation lies in a relatively high energy region. In this article, the double exchange ferromagnet is discussed with the Nagaoka ferromagnetic state of the U = 00 Hubbard model, with respect to the stability of the ferromagnetic state.the stability of the Nagaoka state 5 ) in the case with the infinite repulsion U = 00 around half-filling. In the course of investigations, the importance of the magnonelectron scattering effect has been stressed. 6) -11) On the other side, as an appropriate method to investigate the stability of the ferromagnetic state in the strong coupling at Kokusai Hoken Keikakugaku (UNIV OF TOKYO) on May 16, 2015 http://ptp.oxfordjournals.org/ Downloaded from IS, + 1/2, S, + 1/2) and IS1:' S,), with weights (1x)L and xL, respectively, where the state IS, Sz) satisfies SIS, M) = S(S + l)IS, M) and SzIS, M) = MIS, M). (For the manganese oxides, we have S, = 3/2. The spin 1/2 is due to the conduction electron.) Flipping one spin decreases Sz by LlS z = -1, and then, we will have the extra configurations IS, + 1/2, S, -1/2) and IS" Sf -1), which are respectively obtained from IS, + 1/2, S, + 1/2) and IS" S,}. These, with their weights in one magnon state, are shown in Fig. 1. These configurations are connected by the hopping of the at Kokusai Hoken Keikakugaku (UNIV OF TOKYO) on May 16, 2015 http://ptp.oxfordjournals.org/ Downloaded from J had better be considered within the subspace which do not suffer U.