To provide a gas ion source which can prevent a raw material gas from discharge in its introducing piping when the raw material gas is introduced into the gas ion source mounted in a casing for a high-voltage terminal.

In the gas ion source 3 which is insulated from the ground potential and mounted in the casing 2 for the high-voltage terminal held at a high potential by a high-voltage generating circuit 10, part of the piping 20 for introducing the raw material gas 9 into the gas ion source 3 is made of an insulating material, and a means 27 for applying a magnetic field perpendicular to the flow direction of the raw material gas is provided at the end of the insulating piping 21.