We present the results of C18O observations by the Nobeyama Millimeter Array toward dense clumps with radii of ~ 0.3 pc in six cluster-forming regions including massive (proto)stars. We identified 171 cores, whose radius, line width, and molecular mass range from 0.01 to 0.09 pc, 0.43 to 3.33 km/s, and 0.5 to 54.1 Mo, respectively. Many cores with various line widths exist in one clump, and the index of the line width-radius relationship of the cores and the parental clump differs from core to core in the clump. This indicates that the degree of dissipation of the turbulent motion varies for each core in one clump. Although the mass of the cores increases with the line width, most cores are gravitationally bound by the external pressure. In addition, the line width and the external pressure of the cores tend to decrease with the distance from the center of the clump, and these dependencies may be caused by the inner H2 density structure of the clump that affects the physical properties of the cores. Moreover, the number density of the cores and the number density of young (proto)stars have a similar relationship to the average H2 density of the clumps. Thus, our findings suggest that the cluster is formed in the clump through the formation of such multiple cores, whose physical properties would have been strongly related to the H2 density structure and the turbulent motion of the clump.