PURPOSE: To increase the flow rate of coal slurry in a preheating tube, and prevent the precipitation of the slurry and the blocking of the tube with the slurry, by mixing a specific volume of gas to a coal slurry, and heating the coal slurry.

CONSTITUTION: The coal 6 supplied from the coal hopper 5 through the line 7 is mixed with the solvent supplied through the line 8 to obtain a coal slurry. The slurry-mixing tank 1 is heated at a definite temperature by the steam, etc. supplied through the line 12. The coal slurry transferred from the mixing tank 1 is mixed uniformly by the line 9, the circulation pump 10 and the line 11, and supplied to the slurry preheater 17 in the heating furnace 19 through the line 14, the booster pump 15 and the line 16. In the above process, 2.0W4.0pts.vol. of a gas (preferably H2 gas), based on 1pt.vol. of the coal slurry, is introduced through the line 22 to the preheater 17, and mixed with the coal slurry to heat the slurry.