  Isoscalar monopole strength function in $^{16}$O up to $E_{x}\simeq40$ MeV is
discussed. We found that the fine structures at the low energy region up to
$E_{x} \simeq 16$ MeV in the experimental monopole strength function obtained
by the $^{16}$O$(\alpha,\alpha^{\prime})$ reaction can be rather satisfactorily
reproduced within the framework of the $4\alpha$ cluster model, while the gross
three bump structures observed at the higher energy region ($16 \lesssim E_{x}
\lesssim 40$ MeV) look likely to be approximately reconciled by the mean-field
calculations such as RPA and QRPA. In this paper, it is emphasized that two
different types of monopole excitations exist in $^{16}$O; one is the monopole
excitation to cluster states which is dominant in the lower energy part ($E_{x}
\lesssim 16$ MeV), and the other is the monopole excitation of the mean-field
type such as one-particle one-hole ($1p1h$) which {is attributed} mainly to the
higher energy part ($16 \lesssim E_{x} \lesssim 40$ MeV). It is found that this
character of the monopole excitations originates from the fact that the ground
state of $^{16}$O with the dominant doubly closed shell structure has a duality
of the mean-field-type {as well as} $\alpha$-clustering {character}. This dual
nature of the ground state seems to be a common feature in light nuclei.
