A submerged plasma generator, comprising: a reactor which has a flow path formed in a lengthwise direction therein, the flow path permitting working fluid to travel therein; and a dielectric insert which is disposed in the flow path so as to define the flow path into a first flow path and a second flow path, and has formed therein a through-hole smaller than the flow path to generate micro-nano bubbles by cavitation in the working fluid fed into the first flow path, and includes, on one side thereof, a metallic catalyst arranged to undergo friction with the working fluid flowing through the through-hole so as to release electric charges of the same polarity to the micro-nano bubbles to collapse the micro-nano bubbles and generate plasma; wherein the second flow path in which the working fluid ionized by exposure to the plasma travels is formed in an oval structure, wherein the submerged plasma generator further comprises an ion separation unit which is provided on an outer surface of the reactor corresponding to the second flow path, and is configured to form a magnetic field in the second flow path to separate H+ ions and OH- ions from the working fluid which is ionized by exposure to the plasma by passing through the dielectric insert, wherein a contact surface of the ion separation unit which is in contact with the outer surface of the reactor corresponds to a diameter of the second flow path or is formed to have a width larger than the diameter of the second flow path, wherein a seating support groove in which the ion separation unit is seated is formed on the outer surface of the reactor to be dented with a predetermined depth toward the flow path and the contact surface which is in contact with the seating support groove and an inner surface of the reactor which forms the second flow path are spaced apart from each other with a distance (D') of at least 4 to 11 mm therebetween. a reactor which has a flow path formed in a lengthwise direction therein, the flow path permitting working fluid to travel therein; and a dielectric insert which is disposed in the flow path so as to define the flow path into a first flow path and a second flow path, and has formed therein a through-hole smaller than the flow path to generate micro-nano bubbles by cavitation in the working fluid fed into the first flow path, and includes, on one side thereof, a metallic catalyst arranged to undergo friction with the working fluid flowing through the through-hole so as to release electric charges of the same polarity to the micro-nano bubbles to collapse the micro-nano bubbles and generate plasma; wherein the second flow path in which the working fluid ionized by exposure to the plasma travels is formed in an oval structure, wherein the submerged plasma generator further comprises an ion separation unit which is provided on an outer surface of the reactor corresponding to the second flow path, and is configured to form a magnetic field in the second flow path to separate H+ ions and OH- ions from the working fluid which is ionized by exposure to the plasma by passing through the dielectric insert, wherein a contact surface of the ion separation unit which is in contact with the outer surface of the reactor corresponds to a diameter of the second flow path or is formed to have a width larger than the diameter of the second flow path, wherein a seating support groove in which the ion separation unit is seated is formed on the outer surface of the reactor to be dented with a predetermined depth toward the flow path and the contact surface which is in contact with the seating support groove and an inner surface of the reactor which forms the second flow path are spaced apart from each other with a distance (D') of at least 4 to 11 mm therebetween.