Provided is a negative ion irradiation device in which an object is irradiated with a negative ion. The device includes a chamber that allows the negative ion to be generated therein, a gas supply unit that supplies a gas which is a raw material for the negative ion, a plasma generating portion that generates plasma, a voltage applying unit that applies a voltage to the object, a control unit that performs control of the gas supply unit, the plasma generating portion, and the voltage applying unit. The control unit controls the gas supply unit to supply the gas into the chamber, controls the plasma generating portion to generate the plasma in the chamber and to generate the negative ion by stopping the generation of the plasma, and controls the voltage applying unit to start voltage application during plasma generation and to continue voltage application after plasma generation stop.