A liquid treatment apparatus comprising: a tubular treatment tank of which a sectional shape orthogonal to a central axis is a circular shape; an insulating space forming member which is disposed at a side of one end of the central axis 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 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 is generated in the treatment tank by introducing the liquid from a tangential direction of the circular shape of the treatment tank, wherein due to the swirling flow, the pressure in the vicinity of the central axis drops to the saturated water vapor pressure or less, so that a portion of the liquid vaporizes and a gas phase is generated in the swirling flow of the liquid in the vicinity of the central axis, 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 is a circular shape; an insulating space forming member which is disposed at a side of one end of the central axis 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 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 is generated in the treatment tank by introducing the liquid from a tangential direction of the circular shape of the treatment tank, wherein due to the swirling flow, the pressure in the vicinity of the central axis drops to the saturated water vapor pressure or less, so that a portion of the liquid vaporizes and a gas phase is generated in the swirling flow of the liquid in the vicinity of the central axis, 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 is generated in the treatment tank by introducing the liquid from a tangential direction of the circular shape of the treatment tank, wherein due to the swirling flow, the pressure in the vicinity of the central axis drops to the saturated water vapor pressure or less, so that a portion of the liquid vaporizes and a gas phase is generated in the swirling flow of the liquid in the vicinity of the central axis, 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.