Abstract: A water purifier comprising a cylinder provided at its lower portion with an air chamber and water sucking holes and adapted to be intermittently supplied with compressed air and water, a honeycombed tube structure including a multiplicity of tubes arranged in a side-by-side relation concentrically around the cylinder and a protecting sleeve for protecting the honeycombed tube structure. The protecting sleeve may be provided on its outer peripheral surface with a buoyancy chamber. A plurality of units of water purifier may be arranged in stages. In some cases, the water purifier is placed to occupy the whole space of a water tank into which the water to be treated is introduced continuously.