In order to attain a main objective of the present invention to provide an ion source capable of efficiently extracting ions, the ion source is configured to include: a conductive tubular body having an ion emitting aperture in a tip surface thereof and a penetration portion in a side wall thereof allowing thermo-electrons to pass through from an outside toward an inside; a mesh surrounding an outer periphery of the penetration portion; and a thermionic emission filament surrounding an outer periphery of the mesh, such that the thermo-electrons emitted from the thermionic emission filament pass through the mesh and reach the inside of the conductive tubular body through the penetration portion.