Natural inflation is a large field model, which is favored from the view point of the initial condition problem.The inflaton can be understood as a Nambu Goldstone boson associated with a spontaneous breaking of an U(1) symmetry. The inflaton potential can be understood as an explicite breaking of the U(1) symmetry to a discrete one. Since the decay constant of the inflaton must be larger than the Planck scale for a successful slow-roll inflation, it is not clear whether natural inflation model can be treated by four dimensional field theory, which has a cut off at the Planck scale. In this talk, I show that the large decay constant can be obtained from a field theory with all the scale below the Planck scale, by considering hierarchical U(1) charges.