An ion generator includes: an arc chamber which defines a plasma generation space; a cathode which emits thermoelectrons toward the plasma generation space; and a repeller which faces the cathode with the plasma generation space interposed therebetween. The arc chamber includes a box-shaped main body on which a front side is open, and a slit member which is mounted to the front side of the main body and provided with a front slit for extracting ions. An inner surface of the main body which is exposed to the plasma generation space is made of a refractory metal material, and an inner surface of the slit member which is exposed to the plasma generation space is made of graphite.