A scale trapping unit includes a casing having a water inlet and a water outlet at respective ends, and a scale adsorbing element loaded in the casing, the scale adsorbing element having opening ports, each of the opening ports having a representative length greater than a length of a scale particle. The casing has a void region at least at the end where the water inlet is located. A scale trapping unit inlet pipe is perpendicularly connected to a central portion of a bottom surface at the end of the casing where the water inlet is located. The scale trapping unit inlet pipe, the void region, and the scale adsorbing element are linearly aligned so that water flows therethrough in a constant direction.