The exchanging device is for reducing hardness of hard water. Raw water goes through the exchanger from top to bottom, to be softened by the sodium ion exchanger inside the device. Magnesium and calcium that contained by water are exchanged with that of sodium. Then the raw water becomes soft water containing very few ions of magnesium and calcium. The soft water can be used for feeding water of boiler and for some industrial uses. Due to its high dissolving ability in water, sodium ions will not form hard sediment in the boiler and can be easily removed. Raw water shall be filtered and settled with less suspended solids before coming into the exchanging device. After being softened in the device, the water?s hardness shall be less than 0.03mg equivalent.