According to one embodiment of the present invention, an ion source includes: an anode tube in which gas flowing in through one side is ionized and discharged to the other side and in which a slit is formed on the outer circumference thereof; a filament which emits thermal electrons toward the slit so as to ionize the gas; and a diffusion-preventing body arranged between the filament and the slit and having at least one hole through which the thermal electrons can pass so as to reduce the diffusion of the thermal electrons flowing into the anode tube.