Acidic oral compositions having reduced tooth erosion characteristics, especially acid beverages such as fruit juice drink concentrates, or oral healthcare products such as mouthwashes, are prepared by adding a calcium compound to the acid composition so that the mol ratio of calcium to acid ranges from 0. The ceramic insulator has an engagement portion which is engaged with a receiving surface of the protrusion and an intermediate trunk portion extending rearward from the rear end of the engagement portion. The organic gas impurities in the ammonia gas are removed by activated charcoal, and the oxygen is removed by deoxidizer, and then the high-purity ammonia gas is gotten.