and the next nearest neighbor transfer energy $t'$. Recently, appropriate values of $t'$ were found to remarkably enhance SC correlations \[\]. We apply the variational Monte Carlo method \[\] mainly to the $10 \times 10$-site system with electron numbers from 68 to 92. This method has a merit that it allows to treat larger values of $U$ than the quantum M.C. methods. We revisit the issue investigated in ref. \[\], carrying out more extensive study with higher precision in a wider parameter space. In a previous preliminary report \[\] we worked out a variational Monte Carlo calculation 84 electrons on the $10 \times 10$ lattice and confirmed that the 2D Hubbard model has the $d$-wave SC state. In the present work first we examine the $U$-dependence of the energy gain due to the condensation into the SC state and find the optimal value of $U$ is about $8$ in units of the nearest neighbor transfer energy $t$. In the previous report we confirmed that the energy gain in the SC state sharply increases with the introduction of $t'$ with a negative value. In this report we calculate the energy gain as a function of $t'$ in a sufficiently wide range 