A method for dewatering a water-containing combustible solid excluding coal, comprising heating, in a sealed vessel, the water-containing combustible solid at a temperature of 100° C. to 300° C. under a pressure not less than a saturated steam pressure at said heating temperature, while applying a shearing force of 0.01 MPa to 20 MPa to said water-containing combustible solid. The method allows the production of a slurry in water which contains the water separated from the water-containing combustible solid and a combustible solid from which the water is removed, has a suitable viscosity and water content, and can be subjected to spray combustion, at low costs.