PURPOSE: To inexpensively produce cement clinker by adding predried fuel of a low cost as secondary fuel at a furnace inlet and burning this fuel in a rotary cylindrical furnace.

CONSTITUTION: This process for producing the cement clinker consists in charging paper sludge predried to a moisture content of about 30 wt.% as the secondary fuel into the rotary cylindrical furnace 10 by the waste gases of the furnace. A part of the waste gases from the rotary cylindrical furnace 10 are supplied as a main stream into a pulverizing and drying device 11 and another part are supplied to the drying device 5 of the paper sludge. The waste gases discharged from the drying device 5 are supplied to the main stream of the waste gases described above after passage through a paper remaining amt. separator 23.