  We study the compositeness of near-threshold quasi-bound states in the
framework of effective field theory. From the viewpoint of the low-energy
universality, we revisit the model-independent relations between the structure
of the bound state and the observables in the weak binding limit. The effective
field theory is utilized to generalize the weak-binding relation of the stable
bound states to unstable quasi-bound states with decay modes. We present the
interpretation of the complex values of the compositeness for the unstable
states. Combining the model-independent relation and the threshold observables
extracted from the experimental data, we show that Lambda(1405) is dominated by
the Kbar N molecular structure and that a_0(980) is dominated by the non-Kbar K
component.
