An H3+ ion is used as an ion beam to achieve improvement in focusing capability influencing observed resolution and machining width, improvement in the beam stability, and a reduction in damage to the sample surface during the beam irradiation, in the process of observation and machining of the sample surface by the ion beam. The H3+ ion can be obtained by use of a probe current within a voltage range 21 around a second peak 23 occurring when an extracted voltage is applied to a needle-shaped emitter tip with an apex terminated by three atoms or less, in an atmosphere of hydrogen gas.