PURPOSE: To provide an ECR type ion source in which the fouling of the inner wall of an insulator is effectively prevented to extend the life until the withstand voltage is lowered, and a discharge is difficult to generate between a drawing electrode system and a high voltage-side shield cylinder.

CONSTITUTION: An insulator 26 is formed in two stages, a ring electrode supporting flange 32 is nipped between the both, and a high voltage-side shield cylinder 28 and a cylindrical intermediate electrode 34 smaller in diameter than a ground-side shield cylinder 30 are mounted on the inside thereof. The intermediate electrode 34 is mounted on the electrode supporting flange 32 so as to form a labyrinth structure in which the inner wall of the insulator 26 is never directly seen from an ion beam 22. Potential dividing resistances 36 are connected between the electrode supporting flange 32 and a high voltage- side ion source chamber 12 and between it and a ground potential-side flange part 24, respectively.