According to one embodiment, there is provided an ion source. The ion source includes a vacuum-exhausted vacuum chamber, a target which is set in the vacuum chamber and generates a plurality of valences of ions by irradiation of a laser beam, an acceleration electrode which is applied with voltage in order to accelerate the ions generated by the target, and an intermediate electrode which is provided between the target and the acceleration electrode and is applied with reverse voltage of the voltage applied to the acceleration electrode.