A submerged plasma generator includes: a reactor inside of which a flow path, through which a working fluid passes, is formed along a lengthwise direction; and a dielectric insert which is disposed in the flow path so as to define the flow path into one space and the other space, and has formed therein a through-hole to generate micro-nano bubbles by cavitation in the working fluid fed into the one space of the flow path, and includes, a metallic catalyst which undergoes friction with the working fluid flowing through the through-hole and releases electric charges of the same polarity to the micro-nano bubbles to collapse the micro-nano bubbles and generate plasma; in which the other space of the flow path in which the working fluid ionized by exposure to the plasma travels is formed in an oval structure.