  We investigate a novel type of Langevin model that describes the
nonequilibrium dynamics of a classical particle interacting with a spatially
extended environment. In this model, a particle, which interacts with the
environment through the nonlinear interaction Hamiltonian, is driven by a
constant external force, and subsequently, it reaches a nontrivial
nonequilibrium steady state. We derive an effective Langevin equation for the
particle in the nonequilibrium steady states. Using this equation, we calculate
the effective temperature defined as the ratio of the correlation function of
the velocity fluctuation to the linear response function with respect to a
small perturbation. As a result, it is shown that the effective temperature
associated with the time scale of the particle is identical to the kinetic
temperature if the time scale of the environment and that of the particle are
well separated. Furthermore, a noteworthy expression, which relates the kinetic
temperature with the curvature of the driving force-mean velocity curve, is
derived.
