$0451.6-0305$, and 2.29 arcmin$^{-2}$ for $R_{c}-K' \geq$ 5.3 in MS $0440.5+0204$ at $K'=21.6$ for the source plane at $z_{\rm{source}} = 1.5$. The cumulative surface densities as a function of the $K'_{\rm{source}}$ magnitude for a sample with $R_{\rm{c}}-K' \geq 5.3$ are shown in figure 9. We also plotted the result for the 0.6 arcmin$^2$ area in MS $0451.6-0305$ (rectangle surrounded by dotted lines in figure 3), in which 7 EROs were detected. About 20 and 10% of the whole survey areas reach to a depth of $K'_{\rm{lim}} \sim 23.0$, when we adopt the detection limit as $K'=22.0$. Because of the limit by the depth of the $R$-band magnitudes, the depth of our “ERO samples” are decreased to $K_{\rm{source}}\sim22.0$. Our sample thus provides one of the deepest ERO sample ever reported. It is noted that we obtained mostly the same results using other model parameters used in Wu et al. (1998) and Williams et al. (1999).

For a comparison, we overplotted the result of G.P. Smith et al.(2002) and Wehner et al.(2002) without the sample of A 2390 in figure 9. Both ERO samples were made by targeting lensing cluster fields, and selected by the same color criterion as ours ($R_{\rm{c}}-K'>5.3$). We 