  We present nonlinear conduction phenomena in the Mott insulator Ca2RuO4
investigated with a proper evaluation of self-heating effects. By utilizing a
non-contact infrared thermometer, the sample temperature was accurately
determined even in the presence of large Joule heating. We find that the
resistivity continuously decreases with currents under an isothermal
environment. The nonlinearity and the resulting negative differential
resistance occurs at relatively low current range, incompatible with
conventional mechanisms such as hot electron or impact ionization. We propose a
possible current-induced gap suppression scenario, which is also discussed in
non-equilibrium superconducting state or charge-ordered insulator.
