Simultaneous measurements of the ac susceptibility and the dc magnetization of an untwinned single-crystal ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ have been carried out to clarify the nature of vortex lattice melting. The first-order melting transition appears as a jump in the dc magnetization. Evidence for effectual pinning at temperatures above the melting transition is obtained as a sharp transition in ${\mathrm{\ensuremath{\chi}}}^{\ensuremath{'}}$ and a sharp peak in ${\mathrm{\ensuremath{\chi}}}^{\ensuremath{'}\ensuremath{'}}$ of width 0.2 K in 10 kG. In addition, a precursor lattice softening phenomenon can be seen in ${\mathrm{\ensuremath{\chi}}}^{\ensuremath{'}}$ and ${\mathrm{\ensuremath{\chi}}}^{\ensuremath{'}\ensuremath{'}}$ at temperatures below the melting temperature. This sequential change is more visible when the ac susceptibility is given in the ${\mathrm{\ensuremath{\chi}}}^{\ensuremath{'}}$--${\mathrm{\ensuremath{\chi}}}^{\ensuremath{'}\ensuremath{'}}$ representation. It has been confirmed that there exists a melting transition of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ even below 10 kG.