The ac response of type-II superconductors to an alternating magnetic field is numerically studied on the basis of the time-dependent Ginzburg - Landau equations. We examine the temperature dependence of the ac susceptibility associated with a small ac magnetic field in the absence of a bias dc field. It is shown that with increasing temperature the in-phase component of the fundamental susceptibility exhibits a step-like change from a negative constant value to zero, while the out-of-phase component of the fundamental susceptibility and the third-harmonic component have a peak at a certain temperature near the superconducting transition temperature. These results are in qualitative agreement with those of recent experiments on high- superconductors.