To provide an ion source of compact device constitution in which the operating time for forming plasma can be prolonged in the ion source for forming the plasma of high density, and provide a preparation method of a filament having a long service life which is used for this ion source.

In the preparation of the filament which emits a thermal electron used for the ion source, and after the diameter of a thermal electron emitting part of filament wire materials has been enlarged by thermal metal spraying in a state that both end parts of the filament wire materials are coated, both end parts of the filament wire materials not thermally metal-sprayed are attached to a plasma forming container of the ion source as a filament retaining part.

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