To prevent the clogging of a pipeline with Mg(NH4)PO4 (MAP) particles by preliminarily removing an MAP particle generating main cause component bringing about trouble such as the clogging of the pipeline caused by the adhesion of MAP particles.

An aeration device 6 is provided on the upstream side of a pipeline 5 for transporting sludge and sludge is preliminarily aerated in the aeration device 6 to form Mg(NH4)PO4 (MAP) particles in sludge and an MAP particle generating main cause component, that is, phosphate ions or magnesium ions are removed before sludge is sent into the pipeline 5. Since the concn. of the MAP particle generating main cause component in sludge is reduced by this constitution, a fear such that MAP particles adhere to the inner wall of the pipeline 5 to bring about trouble such clogging can be avoided.