  We investigated the formation and evolution of CDM subhalos in galaxy-sized
and cluster-sized halos by means of N-body simulations. Our aim is to make
clear what the ``dwarf galaxy problem'' is. It has been argued that the number
of subhalos in simulated galaxy-sized halos is too large compared to the
observed number of dwarfs in the local group, while that in cluster-sized halos
is consistent with observed number of galaxies in clusters such as the Virgo
cluster. We simulated nine halos with several different mass resolutions and
physical scales. We found that the dependence of the cumulative number of
subhalos N_c on their maximum circular velocity V_c is given by N_c \propto
V_c^-3, down to the reliability limit, independent of the mass of the main
halo. This implies that simulations for cluster-sized halos give too many halos
with V_c ~ 140km/s or less. Previous comparisons of cluster-sized halos gave
much smaller number of subhalos in this regime simply because of their limited
resolution. Our result implies that any theory which attempts to resolve the
missing dwarf problem should also explain the discrepancy of the simulation and
observation in cluster-sized halos.
