A plasma generator (1) comprises: a liquid containing part (3) which contains a liquid (6) including water; a gas containing part (4) which contains a gas; and a partition wall part (5) that separates the liquid containing part and the gas containing part from each other and is provided with a gas passage (5a) through which the gas in the gas containing part is led to the liquid containing part. The plasma generator (1) is also provided with a first electrode (10) that is arranged in the gas containing part and a second electrode (11) that is arranged so as to be in contact with the liquid in the liquid containing part. The plasma generator (1) is further provided with: a gas supply unit (9) which supplies the gas to the gas containing part; a plasma power supply unit (13); and a liquid inflow prevention device (a control unit (14)) which prevents the liquid from flowing into the gas containing part from the liquid containing part through the gas passage.