The exchanging device is for reducing the hardnes of hard water. Raw water goes through the exchanger from top to bottom, to be softened by the sodium iomexchanger inside the device. The positve ions of magneslum and calcium are exchanged with that of sodium. Then the raw water becomes soft water contiaing very few ions of magnesium and calcium. The soft water can be used for feeding water of boiler and for some hard sediment in the boiler and can be easily removed. Raw water shall be filtered and settled with almost on suspended before coming into the lonechaging device. After being softened in the device, the water shardness shall be less than 0.03mg equiwalent/1.