NOVELTY - A waste-heat-recovery device consists of a depressing vessel which depresses solid and liquid slurry mixed with solid production and liquid production produced from heated and pressured organic sludge, a reservoir which stores reaction product of solid and liquid slurry depressed and cooled in the depressing vessel, a separator, a dryer, an anaerobic digestion reaction tank, and a waste heat recovering assembly. The temperature of depressing vessel is higher than that of the reservoir.    USE - Device is used for recovering waste heat from solid-liquid slurry of organic sludge (claimed).    DETAILED DESCRIPTION - A waste-heat-recovery device consists of a depressing vessel which depresses solid and liquid slurry mixed with solid production and liquid production produced from heated and pressured organic sludge, a reservoir which is piped with the depressing vessel and stores reaction product of solid and liquid slurry depressed and cooled in the depressing vessel, a separator which is piped with the reservoir and separates solid and liquid from reaction product of solid and liquid slurry periodically provided from the reservoir in order to let it adjust addition amount, a dryer which is connected with the separator and dries solid production separated from the separator by receiving heat generated from solid and liquid slurry in the depressing vessel, an anaerobic digestion reaction tank which is piped with the separator and generates biogas through anaerobic digestion of the liquid product separated from the separator by receiving heat generated from the reaction product in the reservoir, and a waste heat recovering assembly which connects between the depressing vessel and the drier and between the reservoir and the anaerobic digestion reaction tank and recovers heat generated from the depressing vessel and the reservoir to the dryer and the anaerobic digestion reaction tank. The temperature of depressing vessel is higher than that of the reservoir. An INDEPENDENT CLAIM is included for waste-heat recovery method.