We measured the fluctuations of a lyotropic liquid crystal lamellar (L-alpha) phase in dilute surfactant concentration and a sponge (L-3) phase by dynamic light scattering technique. We observed the concentration fluctuation induced by the thermal undulation of layers in the L-alpha phase and obtained the dependence of the effective rigidity k(c) of layers on the repeat distance d, which reflected the fluctuation-induced layer softening. In the L-3 phase, two kinds of fluctuation modes were observed.