A liquid treatment apparatus comprising: a dielectric tube through which water to be treated flows; a first electrode at least a part of which is disposed in the dielectric tube; a second electrode at least a part of which is disposed in the dielectric tube, characterized in that the second electrode is at a position upstream of the first electrode such that the water to be treated flows from the second electrode toward the first electrode; a gas supplier is configured to supply a gas for generating a bubble that covers a conductor-exposed portion of the first electrode; and a power supply configured to apply a voltage between the first electrode and the second electrode in a state in which the conductor-exposed portion of the first electrode is covered by the bubble, wherein a longitudinal direction of the first electrode crosses a direction in which the water to be treated flows from the second electrode toward the first electrode in the dielectric tube, the liquid treatment apparatus further comprising: an insulator (106, 206) disposed in the dielectric tube so as to form a space (108, 208) around the first electrode, the insulator (106, 206) having an opening (107, 207) through which the space (108, 208) is connected to the dielectric tube, wherein the gas supplier is configured to generate the bubble in the water to be treated from the opening (107, 207) in the insulator (106, 206) by supplying the gas to the space (108, 208), and the power supply is configured to cause an electrical discharge in the bubble and generates plasma by applying the voltage between the first electrode and the second electrode. a dielectric tube through which water to be treated flows; a first electrode at least a part of which is disposed in the dielectric tube; a second electrode at least a part of which is disposed in the dielectric tube, characterized in that the second electrode is at a position upstream of the first electrode such that the water to be treated flows from the second electrode toward the first electrode; a gas supplier is configured to supply a gas for generating a bubble that covers a conductor-exposed portion of the first electrode; and a power supply configured to apply a voltage between the first electrode and the second electrode in a state in which the conductor-exposed portion of the first electrode is covered by the bubble, wherein a longitudinal direction of the first electrode crosses a direction in which the water to be treated flows from the second electrode toward the first electrode in the dielectric tube, the liquid treatment apparatus further comprising: an insulator (106, 206) disposed in the dielectric tube so as to form a space (108, 208) around the first electrode, the insulator (106, 206) having an opening (107, 207) through which the space (108, 208) is connected to the dielectric tube, wherein the gas supplier is configured to generate the bubble in the water to be treated from the opening (107, 207) in the insulator (106, 206) by supplying the gas to the space (108, 208), and the power supply is configured to cause an electrical discharge in the bubble and generates plasma by applying the voltage between the first electrode and the second electrode. an insulator (106, 206) disposed in the dielectric tube so as to form a space (108, 208) around the first electrode, the insulator (106, 206) having an opening (107, 207) through which the space (108, 208) is connected to the dielectric tube, wherein the gas supplier is configured to generate the bubble in the water to be treated from the opening (107, 207) in the insulator (106, 206) by supplying the gas to the space (108, 208), and the power supply is configured to cause an electrical discharge in the bubble and generates plasma by applying the voltage between the first electrode and the second electrode.