[en] An ionization sensor, which ionizes iodine vapor on a heated filament and collects ionized iodine on a collector at positive potential, was made on an experimental basis. Iodine vapor in rare gas was determined with using the sensor on the line and the characteristic of the sensor was examined. Iodine vapor was generated from iodine crystals in an iodine evaporator and transferred to the sensor with carrier-rare gas. The iodine vapor was continuously monitored by the sensor and trapped in solution of sodium hydroxide. The amount of iodine in the solution was determined by chemical analyses. The integrated value of ion current was compared with the collected amount of the iodine. A proportional relation is observed between the collected amount and the integrated value obtained from the sensor with a platinum collector, but not found between the amount and the value obtained from the sensor with a stainless steel collector. (author)