A polarity control unit includes a flag storage unit that stores any one of a first polarity pattern by which a positive direct-current voltage is applied to discharge needles in a first group and a negative direct-current voltage is applied to discharge needles in a second group and a second polarity pattern by which a negative direct-current voltage is applied to the discharge needles in the first group and a positive direct-current voltage is applied to the discharge needles in the second group, a flag update unit that rewrites one flag stored in the flag storage unit to another flag when a power switch is turned on from its off state or is turned off from its on state, and a command unit that commands a polarity pattern corresponding to the flag stored in the flag storage unit to be output from the polarity output unit.