A distance D between a beam drawing electrode 3, which provides a minimum beam divergence angle when the divergence of beam caused by the space charge is not considered, and an arc chamber 2 is found from a theory and a numeral expression described by Junzo Ishikawa, in Ion Source Engineering, pp.178-186 published by Ionics Kabushiki Kaisha as follows. ##EQU1## where S is an area of a drawing ho