The internal structure of exotic hadron candidates are studied from the viewpoint of the compositeness of near-threshold states.
We focus on the Weinberg's weak-binding relation between the experimental observables and the compositeness of the state.
First we extend the relation for quasibound states within the framework of the effective field theory.
Next, considering the correction term, we develop a systematic method to estimate the uncertainty of compositeness in the weak-binding relation.
Finally, applying the extended relation and the error evaluation method, we conclude that the structure of Lambda(1405) is dominated by the Kbar N component.