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Isoscalar monopole strength function in $^{16}$O up to $E_{x}\simeq40$ MeV isdiscussed. We found that the fine structures at the low energy region up to$E_{x} \simeq 16$ MeV in the experimental monopole strength function obtainedby the $^{16}$O$(\alpha,\alpha^{\prime})$ reaction can be rather satisfactorilyreproduced within the framework of the $4\alpha$ cluster model, while the grossthree bump structures observed at the higher energy region ($16 \lesssim E_{x}\lesssim 40$ MeV) look likely to be approximately reconciled by the mean-fieldcalculations such as RPA and QRPA. In this paper, it is emphasized that twodifferent types of monopole excitations exist in $^{16}$O; one is the monopoleexcitation 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-fieldtype such as one-particle one-hole ($1p1h$) which {is attributed} mainly to thehigher energy part ($16 \lesssim E_{x} \lesssim 40$ MeV). It is found that thischaracter of the monopole excitations originates from the fact that the groundstate of $^{16}$O with the dominant doubly closed shell structure has a dualityof the mean-field-type {as well as} $\alpha$-clustering {character}. This dualnature of the ground state seems to be a common feature in light nuclei.

Isoscalar monopole excitations in $^{16}$O: $\alpha$-cluster states at low energy and mean-field-type states at higher energy