[en] Spontaneous magnetic fields due to the temperature gradient nabla T₀ produced by a focussed laser beam on one point of a pellet are taken into account in deriving the dispersion relation of Rayleigh-Taylor instability. Growth rate γ decreases with time. Density fluctuation with wavelength shorter than 1.5(R/L sub(T)) x (n sub(s)/n₀)sup(1/2) μm is remarkably stabilized, where R, L sub(T), n sub(s) and n₀ are the radius of a pellet, L sub(T)sup(-1) = ⁺nabla T₀/T₀⁺, number densities of solid and the pellet. Validity condition of the theory is γt₀ >> 1 or in another form R >> L, where t₀ is the time of thermal expansion of a pellet and L^-1 = ⁺nabla n₀/n₀⁺. (author)