Decay of the acoustic motions in an early stage of the hot universe is studied in detail. Considering a growth of density perturbation by gravitation and its decay by dissipative processes, i.e. viscosity and thermal conductivity, we can get a gross feature of the size spectrum of density inhomogeneity at the stage of recombination of the cosmic plasma. The size spectrum has a flat maximum between 1012-13M⊙ and 1017-18M⊙. After the time of decoupling, the density perturbation evolves into a galaxy or a cluster of galaxies by the hydrodynamical instability. The maximum mass formed by this mechanism is given as 1014-15M⊙ assuming the density perturbation δρ/ρ≃10-1.