A liquid treatment apparatus comprising: a tubular treatment tank of which a sectional shape orthogonal to a central axis (X1) is a circular shape; an insulating space forming member which is disposed at a side of one end of the central axis (X1) of the treatment tank and has a through-hole and a space connected to the treatment tank via the through-hole ; a first electrode which is disposed in contact with the space ; a second electrode which is disposed on a side of the other end of the central axis (X1) of the treatment tank ; a power supply which applies a pulse voltage between the first electrode and the second electrode ; a liquid introduction port at one end of the treatment tank disposed so as to introduce a liquid from a tangential direction of the treatment tank , and a discharge unit disposed at the other end of the treatment tank ; wherein the liquid treatment apparatus is configured so that: a swirling flow (F1) is generated in the treatment tank by introducing the liquid (L1) from a tangential direction of the circular shape of the treatment tank , wherein due to the swirling flow (F1), the pressure in the vicinity of the central axis (X1) drops to the saturated water vapor pressure or less, so that a portion of the liquid (L1) vaporizes and a gas phase is generated in the swirling flow of the liquid (L1) in the vicinity of the central axis (x1), said gas phase reaching the space and the through-hole ; - the pulse voltage is applied to the generated gas phase to generate plasma in the gas phase, thereby causing the liquid to generate a treatment liquid. a tubular treatment tank of which a sectional shape orthogonal to a central axis (X1) is a circular shape; an insulating space forming member which is disposed at a side of one end of the central axis (X1) of the treatment tank and has a through-hole and a space connected to the treatment tank via the through-hole ; a first electrode which is disposed in contact with the space ; a second electrode which is disposed on a side of the other end of the central axis (X1) of the treatment tank ; a power supply which applies a pulse voltage between the first electrode and the second electrode ; a liquid introduction port at one end of the treatment tank disposed so as to introduce a liquid from a tangential direction of the treatment tank , and a discharge unit disposed at the other end of the treatment tank ; wherein the liquid treatment apparatus is configured so that: a swirling flow (F1) is generated in the treatment tank by introducing the liquid (L1) from a tangential direction of the circular shape of the treatment tank , wherein due to the swirling flow (F1), the pressure in the vicinity of the central axis (X1) drops to the saturated water vapor pressure or less, so that a portion of the liquid (L1) vaporizes and a gas phase is generated in the swirling flow of the liquid (L1) in the vicinity of the central axis (x1), said gas phase reaching the space and the through-hole ; - the pulse voltage is applied to the generated gas phase to generate plasma in the gas phase, thereby causing the liquid to generate a treatment liquid. a swirling flow (F1) is generated in the treatment tank by introducing the liquid (L1) from a tangential direction of the circular shape of the treatment tank , wherein due to the swirling flow (F1), the pressure in the vicinity of the central axis (X1) drops to the saturated water vapor pressure or less, so that a portion of the liquid (L1) vaporizes and a gas phase is generated in the swirling flow of the liquid (L1) in the vicinity of the central axis (x1), said gas phase reaching the space and the through-hole ; - the pulse voltage is applied to the generated gas phase to generate plasma in the gas phase, thereby causing the liquid to generate a treatment liquid. - the pulse voltage is applied to the generated gas phase to generate plasma in the gas phase, thereby causing the liquid to generate a treatment liquid. <EOS>