  In de Sitter space, scale invariant fluctuations give rise to infrared
logarithmic corrections to physical quantities, which eventually spoil
perturbation theories. For models without derivative interactions, it has been
known that the field equation reduces to a Langevin equation with white noise
in the leading logarithm approximation. The stochastic equation allows us to
evaluate the infrared effects nonperturbatively. We extend the resummation
formula so that it is applicable to models with derivative interactions. We
first consider the nonlinear sigma model and next consider a more general model
which consists of a noncanonical kinetic term and a potential term. The
stochastic equations derived from the infrared resummation in these models can
be understood as generalizations of the standard one to curved target spaces.
