An electrically conductive heat-blocking plate 11 with an opening 12 for allowing thermions to pass through is provided between a filament 3, whose temperature can be as high as 2000° to 3000° C., and an ionization chamber 2. The heat-blocking plate 11 is thermally connected via an aluminum block 10 to a heater for maintaining the ionization chamber 2 within a range temperature from 200° to 300° C., and also electrically set at a ground potential, which is approximately equal to the potential of the ionization chamber 2. The heat-blocking plate 11 blocks the radiation heat that the filament 3 emits when energized. Thus, the wall of the ionization chamber 2 is prevented from being locally heated to an abnormally high temperature. As a result, the inner space of the ionization chamber 2 is maintained at an approximately uniform temperature, and the noise due to the decomposition of a metallic material by abnormal heating is prevented. The heat-blocking plate 11 also prevents a thermion-accelerating electric field from penetrating through an electron injection port 5 into the ionization chamber 2 and impeding the extraction of ions produced within the ionization chamber 2. Thus, the ion extraction efficiency is also improved.