A novel nonlinear sigma model method is proposed for the two-dimensional J(1)-J(2) model, which is extended to include plaquette-type distortion. The nonlinear sigma model is properly derived without spoiling the original spin degrees of freedom. The method shows that a single disordered phase continuously extends from a frustrated uniform regime to an unfrustrated distorted regime. By the continuity and Oshikawa's commensurability condition, the disordered ground states for the uniform J(1)-J(2) model are plaquette states with fourfold degeneracy.