Study of topological insulators and superconductors (topological phases of matter) is currently an active research area in condensed-matter physics. One of the most important aspects of them is the renowned relation between bulk and edge states, called the bulk/edge correspondence. The edge state, localized at the material boundary, is sensitive to a boundary condition definitely, but how the boundary condition affects the edge state has not been studied in a systematic way so far. We tackle this problem with a minimal model of topological insulator, and show boundary condition dependence of the edge state, in particular, its energy spectrum and wavefunction behavior. We also argue how such a generic boundary condition is realized in lattice models. This talk is based on a collaboration with K. Hashimoto and X. Wu (Osaka): [arXiv:1509.04676], [arXiv:1602.05577] and work in progress.