An air purifier provides various air purification functions relating to various environmental conditions by increasing a purification function relative to specific contaminants through replacement of a filter based on the environment to be purified. The air purifier includes a main body of a structure allowing air to pass therethrough. A replaceable filter is received into the main body and is replaceable on a basis of the environmental properties of a space to be purified, removing contaminants in the air passing through the main body. Further, the air purifier includes a dust collecting unit to charge dust particles electrically and to collect the dust particles by electrostatic attraction. In addition, a metal filter functions to collect the dust particles charged in the dust collecting unit, and a HEPA filter is used to collect micro-contaminants. The metal filter is positioned in front of the HEPA filter.