  We study implications of the curvaton, a late-decaying light scalar field, on
inflationary cosmology, paying particular attentions to modifications of
observable quantities such as the scalar spectral index of the primordial power
spectrum and the tensor-to-scalar ratio. We consider this issue from a general
viewpoint and discuss how the observable quantities are affected by the
existence of the curvaton. It is shown that the modification owing to the
curvaton depends on class of inflation models. We also study the effects of the
curvaton on inflation models generated by the inflationary flow equation.
