To provide a sludge conditioning method which positively and systematically subjects a sludge layer to consolidative draining and the like, to thereby condition the sludge having soil characteristics including a water content and viscous strength sufficient for excavation, transportation, discarding, or diverted application such as mounding and backfilling.

The water content and the strength of the sludge are set to predetermined values, and a compacting time is set to a predetermined value so as to condition the sludge to the intended soil character (water content, strength, density) in a predetermined time. Then mounding is planarly and stepwise carried out on a drained layer laid on the sludge layer, based on the set values, so that sliding rapture of the sludge layer does not occur caused by the mounding, and the sludge layer is compacted and settled by the load of the mounding, to thereby progressively increase the strength of the sludge layer. In this manner the sludge layer is conditioned to the sludge having the soil characteristics including the water content and viscous strength sufficient for excavation, transportation, discarding, or diverted application such as mounding and backfilling.