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$(alp
ha,alpha^{prime})$ reaction can be rather satisfactorily reproduced within the framework of the $4alpha$ 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.
