48 may be provided the plasma sheath controller 41 or 41 u2032 may be attached to the grid assembly 49 u2032 hereinafter only differences therebetween will be described in brief the third ion extraction grid 48 may be interposed between the second ion extraction grid 45 and the specimen chamber 51 in this case the second ion extraction grid 45 may be interposed between the first ion extraction grid 43 and the third ion extraction grid 48 the other insulating layer 47 may be interposed between the second ion extraction grid 45 and the third ion extraction grid 48 the third ion extraction grid 48 may be a conductor the other grid assembly 49 u2032 may include the first ion extraction apertures 49 h the first ion extraction apertures 49 h may pass through the first ion extraction grid 43 the insulating member 44 the second ion extraction grid 45 the other insulating member 47 and the third ion extraction grid 48 the first ion extraction apertures 49 h may have a first width w 1 the first ion extraction grid 43 the second ion extraction grid 45 and the third ion extraction grid 48 may be arranged with reference to the first ion extraction apertures 49 h a voltage of the same polarity as the first ion extraction grid 43 may be applied to the third ion extraction grid 48 in addition a voltage lower than the first ion extraction grid 43 may be applied to the third ion extraction grid 48 for example when a first positive voltage is applied to the first ion extraction grid 43 a second positive voltage is applied to the third ion extraction grid 48 and the second voltage may be lower than the first voltage the first ion extraction grid.