A pulse voltage having a frequency of 15 kHz is applied from a discharging power supply unit (48) to between a crankshaft (12) and an electrode plate (45) at a current density of 2.5 mA/cm2 to generate a glow discharge and an electric heater (34) is driven at a 40% output (64 kW/kg) to heat the crankshaft (12) to up to 400° C., and then heating is continued with the current density of a glow discharge set at 0.5 mA/cm2 and the output of the electric heater (34) set at 90% (144 kW/kg), thereby effecting nitriding at a desired nitriding temperature.