A high voltage power supply apparatus includes a high voltage direct current voltage source, a power switch configured to apply an output of the high voltage direct current voltage source to process equipment, and a sensing circuit unit including a sensor unit including a sensor and at least one operational amplifier, a reference voltage detection unit connected to a node between the sensor and the at least one operational amplifier, and a digital signal processing unit, wherein the sensing circuit unit is connected to an output terminal through which an output of the high voltage direct current voltage source is applied to the process equipment.