A plasma generator 1 includes a first electrode 12 provided in a gas storage section 5; and a second electrode 13 separated from the first electrode 12 and provided in a manner such that at least the portion coupled with the first electrode 12 is in contact with a liquid 17 in a liquid storage section 4. Electric discharge is caused between the first electrode 12 and the second electrode 13 so as to produce plasma in a gas region in the liquid 17 in the liquid storage section and produce hydroxyl radical from water contained in the liquid 17 and oxygen contained in the gas. A voltage controller 60 controls a voltage applied by a plasma power source 15 depending on conditions.