A helium field ion gun for a focused ion beam system was designed and constructed. The main feature of the ion gun structure is the use of a gas jet nozzle with a tungsten field emitter tip near the nozzle end. Since the helium gas jet is collimated towards the emitter tip, the local pressure in the vicinity of the tip is kept 110 times higher than the value for the location of the ion gauge which is about 20 cm away from the helium jet ion gun. Using this ion gun, current density of 1 μA/sr was obtained although the helium gas flow rate ejected from the nozzle is as small as 1.6×10−3Torr l/s. This result indicates high efficiency of the helium gas supply to the emitter tip surface.