There is provided a charged particle beam apparatus having the function of permitting observation of a sample in a gas atmosphere or in a liquid state, the apparatus being intended to let a dry sample be observed as it is getting saturated with an introduced liquid and to prevent a charged particle beam from getting scattered by an unwanted liquid introduced between a diaphragm and the sample. This invention provides a structure including an inlet-outlet part (300) that brings in and out a desired liquid or gas in the direction of the underside or the side of the sample (6), the structure being arranged so that the sample (6) is irradiated with a primary charged particle beam while the sample (6) and the diaphragm (10) are kept out of contact with each other.