Abstract: Disclosed is a carrier, of which biofilms are appropriately adjustable in thickness. In accordance with an aspect of the present disclosure, there is provided a complex carrier included in one component of a sewage treatment apparatus to allow microorganisms to be attached and grown therein. The complex carrier includes a first carrier having a plural spaces for microorganisms to be attached therein to allow microorganisms to be attached and grown therein; and a second carrier having a structure capable of penetrating or passing through the spaces of the first carrier to adjust the thickness of biofilms formed in the spaces of the first carrier.