A working method of performing beam assist deposition or beam assist etching of a sample comprises irradiating a focused charged particle beam onto a region of the sample, and blowing a predetermined gas through a gas blowing nozzle toward the sample region while the focused charged particle beam passes through a passage in a side portion of the gas blowing nozzle and irradiates the sample region. The passage may be a slot provided in the side portion of the gas blowing nozzle such that the focused charged particle beam passes through an inside of the slot. The slot terminates at one end near a tip of the gas blowing nozzle, and the one end of the slot terminates at and opens into the interior of the gas blowing nozzle.