  We investigate preheating of an inflaton field $\phi$ coupled nonminimally to
a spacetime curvature. In the case of a self-coupling inflaton potential
$V(\phi)=\lambda \phi^4/4$, the dynamics of preheating changes by the effect of
the negative $\xi$. We find that the nonminimal coupling works in two ways.
  First, since the initial value of inflaton field for reheating becomes
smaller with the increase of $|\xi|$, the evolution of the inflaton quanta is
delayed for fixed $\lambda$. Second, the oscillation of the inflaton field is
modified and the nonadiabatic change around $\phi=0$ occurs significantly. That
makes the resonant band of the fluctuation field wider. Especially for strong
coupling regimes $|\xi| \gg 1$, the growth of the inflaton flutuation is
dominated by the resonance due to the nonminimal coupling, which leads to the
significant enhancement of low momentum modes. Although the final variance of
the inflaton fluctuation does notchange significantly compared with the
minimally coupled case, we have found that the energy transfer from the
homogeneous inflaton to created particles efficiently occurs for $\xi<-60$.
