Non-Gaussianity generated in the inflationary scenario with nonminimally coupled inflaton field

We investigate a possibility to restrict various inflationary models with nonminimally coupled inflaton field by future measurement of nonlinear parameter fNL which characterizes the non-Gaussianity in the cosmic microwave background temperature fluctuation. These models are related to the minimally coupled inflationary models by conformal transformation. We show that the curvature perturbation is invariant under the conformal transformation up to the second order. By using this property we show that nonminimal coupling f()R does not produce large non-Gaussianity, and, in particular, the nonlinear parameter takes a narrow range -0.022<fNL<-0.007 in the case of f()=1+ξ2/mP2 with a wide range of parameter ξ.