The oscillation of a gas bubble in liquid under externally applied sinusoidal pressure field is studied. When the applied pressure amplitude is small and the forcing frequency is not in the neighborhood of the resonance frequency of the bubble, the familiar linear results are obtained. When the applied amplitude is not small, there is a tendency to generate both the subharmonic oscillations of the spherical mode and the surface waves on the bubble. The theoretical study is carried out by using both the normal mode analysis and the variational method. It is found that the variational method can lead to approximate solutions in a simpler way than other conventional methods. Experimental implication of the theoretical results is also discussed.