To provide a water purifier in which maintenance costs are extremely lowered compared with a conventional one and the replacement time of an RO membrane element is once every four years to seven years, and the reliability of the apparatus is increased.

A water purifier includes: a front filtration apparatus in which raw water sucked by a pump is filtered; a filter that has a cation exchange resin layer that filters water passing the front filtration apparatus; a reverse osmosis membrane filtration apparatus in which water passing the filter having a cation exchange resin is compressed by a pump to be filtered, wherein silica, and calcium or the like are removed by a cation exchange resin, and thereby the life of an RO membrane is prolonged.