We previously reported about the room air contamination during ventilation study using the TECHNEGAS system. Main cause of the contamination with radioactive gas were leakage through the filter and expiration of the patient after the inhalation. Then, a new type filter was developed to reduce the radioactive leakage. The efficacy and usefulness of the new type filter was examined. The radioactive leakage through the new filter was markedly reduced to 1/6 compared with the old type. But the air resistance of passage was increased over two times of the old type. This resistance is not able to disregard on respiratory physiology. Improvement to reduce the resistance is desirable.