Hollow-core photonic bandgap fibers (PBGFs) with low attenuation and dispersion are expected because they use a micro structured cladding region with a hole with air holes to guide light in a hollow core. Transmission characteristics of the PBGFs are affected by surface modes propagating in core-clad boundary. In this paper, we clarify numerically the effect of surface modes on core modes and the mechanism of coupling between core and surface modes.