A charged particle generator includes a charged particle source provided inside an insulating member which comprises a first box-shaped insulating member having an opening at its top; a second box-shaped insulating member housed inside the first box-shaped insulating member and having an opening at its top, and side walls and a bottom portion which are respectively spaced from side walls and a bottom portion of the first box-shaped insulating member; and an inverted box-shaped insulating member having an opening at its bottom and side walls which are inserted into the space between the side walls of the first and second box-shaped insulating members.