An air diffuser apparatus component in a membrane filtration unit including an air main pipe connected to an aeration blower(s), liquid supply piping that joins the air main pipe in midstream, a branch conduit horizontally connected through a gas supply pipe branched from the air main pipe, and a plurality of air diffusion pipes horizontally to the branch pipe conduit, each diffusion pipe having diffusion holes opening substantially vertically downward, is cleaned. This involves setting a gas flow rate in the branch pipe conduit in a range of 1 m/sec or more and 8 m/sec or less, and a liquid supply rate in the branch pipe conduit in a range from 0.03 L/min/mm2 to 0.2 L/min/mm2. This solid sludge clogging diffusion holes to be efficiently removed with a gas-liquid mixing fluid without stopping for a drainage treatment.