Disclosed is an active constant power supply apparatus, which rectifies power having various intensities and frequencies without using a high capacity condenser for a smoothing circuit which degrades the power factor of a circuit, and supplies constant power to a load. The active constant power supply apparatus includes an AC power supplier for supplying AC power; a rectifying circuit which receives AC power from the AC power supply unit and rectifies the received power; a driving coil connected in series to the load receiving power from the rectifying circuit; a power switch which switches the current passing through the driving coil and the load on/off; a pulse-type driving signal generator connected to a gate terminal of the power switch to control the switching on/off operation of the power switch; a turn-off decider which generates a turn-off signal when the current flowing along the driving coil has a value higher than the designed value, so as to turn off the power switch; and a pulse width controller which measures the control period ranging from the switch-on time of the driving signal generator to the switch-off time of the turn-off decider, and controls the driving pulse width of the driving signal generator such that the driving pulse width coincides with the control period.