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.