for a fixed value of $U = 8$ and show that it has a broad peak around a negative value $t' \sim -0.1$. The approximate peak energy gain in the SC state, i.e., at $t' \sim - 0.1$, is plotted as a function of electron density $\rho$ together with that for $t' = 0$. When $t' \sim -0.1$, the energy gain starts to be finite at about $\rho = 0.68$ and increases fairly smoothly with increase of $\rho$ in the range of $0.76 \leq \rho \leq 0.92$. We carried out calculations in both cases of open and closed shells. The smoothness of the energy gain against $\rho$ indicates that the dependence of the energy gain on the shell structure is weak, in contrast to the case with $t' = 0$, and that the results are already close to the bulk limit. This means that the 2D Hubbard model with $t' \sim -0.1$ has a definite bulk-limit energy gain per site in the SC state. While when $t' = 0$, the energy gain in the closed shell state was found much smaller than that in the open shell state. This indicates that the $10 \times 10$ lattice is not sufficiently large 