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An improved method of calculating the linewidth of a laser oscillator due to phase noise is proposed. The fluctuation in the output amplitude that leads to the phase diffusion is calculated as the amplified thermal and the quantum noise which are delta-correlated in time. The saturated gain distribution along the cavity axis that is consistent with the output coupling at the end surfaces is taken into account in calculating the amplification of the noises. The resulting linewidth formula differs somewhat from the conventional formula.