PURPOSE: To stably extract O+, C+, and CO+ ions and prevent the pollution of a hot cathode by O+, C+, and CO+ ions by feeding the ion source gas mixed with Ar gas to CO2 or O2 or the mixed gas of them at the specific ratio into an ion generation chamber having the hot cathode.

CONSTITUTION: The mixed gas mixed with Ar gas to CO2 or O2 gas or the mixed gas of them is used as the ion source gas 3 fed to the ion source of an ion implanter. The ratio of the Ar gas is set to 20-80vol.% against CO2 or O2 gas. When Ar gas is mixed, Ar ions exist in addition to O+, C+, and CO+ ions in an ion generation chamber 2 kept in the plasma state, contaminants on the inner wall of the ion generation chamber 2 and the surface of an ion extracting electrode 6 are removed via the spattering effect by Ar ions, and more stable O+, C+, and CO+ ions can be extracted.