To provide the technique for treating sludges such as raw sludge and digested sludge to be generated in a sewage treatment plant, bottom sludge and sludge being lees of Shochu (Japanese spirits) by using an inexpensive, safe and easy method.

Powder of tuffaceous sandstone, andesite or andesitic tuff or crushed waste tile, which are troublesome when treated, is fed to a mixer together with sludges and agitated/mixed. The water content of the obtained mixture is decreased. The water content-decreased mixture is granulated, dried and fired to obtain a porous material usable as a filter medium, a cleaning material, a soil improving material, a roadbed material, a substitute of agricultural soil or the like.