To provide an aerator having appropriate strength and durability for a fermenter for organic waste such as sewage sludge.

In an aerator in which air is passed from the bottom part of a fermenter 1 to sewage sludge or the like filled in the fermenter 1 to promote its fermentation, diffusing pipes 5 in the aerator are horizontally installed in the bottom part of the fermenter 1 and are buried in filling material P for aeration and jetting holes for the diffused air are opened to below the diffusing pipes 5 and also as the filling material P, the seed skin and hulls of a plant having about 1 cm2 dimensions and 0.3∼1.0 mm thickness is used. For example, the seed skin and hulls of a ginkgo has appropriate strength and also is hardly rotten.