The behaviour of a gas bubble in a viscous liquid under oscillating pressure is analyzed. The extention of the theory from the inviscid case to the viscous case is easily done with a linearlized theory, though not mathematically rigorous. Two asymptotic time solutions are shown in the cases of uniform interior at low frequencies and nonuniform interior at high frequencies. For the uniform interior, the results of calculation show that for the glycerine the effect of viscosity is unable to be neglected and becomes maximum at frequencies ω=10⁵～10⁶ [Hz] for the initial bubble radius R0=10⁻⁴ [cm]. For water and mercury the viscous effect is negligibly small. And for less than ω= 10⁴ [Hz] the ratio of the amplitude of perturbed radius to perturbed pressure is constant and its value is nearly 1/(3+2W), where W is the Webber number.