  We discuss the large mass hierarchy problem in a braneworld model which
represents our acceleratively expanding universe. The Randall-Sundrum (RS)
model with warped one extra dimension added to flat 4-dimensional space-time
cannot describe our expanding universe. Here, we study instead the de Sitter
thin brane model. This is described by the same action as that for the RS
model, but the 4-dimensional space-time on the branes is $\rm dS_4$. We study
the model for both the cases of positive 5-dimensional cosmological constant
$\Lambda_5$ and negative one. In the positive $\Lambda_5$ case, the
4-dimensional large hierarchy necessitates a 5-dimensional large hierarchy, and
we cannot get a natural explanation. On the other hand, in the negative
$\Lambda_5$ case, the large hierarchy is naturally realized in the
5-dimensional theory in the same manner as in the RS model. Moreover, another
large hierarchy between the Hubble parameter and the Planck scale is realized
by the ${\mathcal O}(10^2)$ hierarchy of the 5-dimensional quantities. Finally,
we find that the lightest mass of the massive Kaluza-Klein modes and the
intervals of the mass spectrum are of order $10^2\,\rm GeV$, which are the same
as in the RS case and do not depend on the value of the Hubble parameter.
