Provided is a charged particle beam apparatus (111) to and from which a diaphragm (101) can be easily attached and detached, and in which a sample (6) can be arranged under vacuum and under high pressure. The charged particle beam apparatus includes: a lens barrel (3) holding a charged particle source (110) and an electron optical system (1,2,7); a first housing (4) connected to the lens barrel (3); a second housing (100) recessed to inside the first housing (4); a first diaphragm (10) separating the space inside the lens barrel (3) and the space inside the first housing (4), and through which the charged particle beam passes; a second diaphragm (101) separating the spaces inside and outside the recessed section (100a) in the second housing (100), and through which the charged particle beam passes; and a pipe (23) connected to a third housing (22) accommodating the charged particle source (110). The first diaphragm (10) is attached to the pipe (23), and the pipe (23) and the third housing (22) can be attached to and detached from the lens barrel (3) in the direction of the optical axis (30). A space (105) surrounded by the first housing (4) and the second housing (100) is depressurized, and the sample (6) arranged inside the recessed section (100a) is irradiated with a charged particle beam.