An in-liquid plasma generation device includes a housing which holds a liquid in an internal space, a gas supply tube which includes an opening in the internal space and discharges a gas into the liquid through the opening, a first electrode which has projecting part projecting into the internal space via the opening from inside of the gas supply tube, the projecting part including a conductor covered by a dielectric, a second electrode which surrounds the projecting part of the first electrode and includes a conductor isolated from the liquid by a dielectric, and a voltage applier which applies a voltage to between the first electrode and the second electrode. A space between the projecting part and the second electrode is a flow passage in which the gas discharged from the opening flows.