A dewatering promoter which comprises a mixture of (A) a fine wood flour activated by a mechanochemical treatment and (B) a highly active cellulose fiber or a product formed by a grinding treatment of the mixture; and a method for producing the dewatering promoter which comprises adding the highly active cellulose fiber to the wood flour, in a ratio of more than 10% relative to the total mass of the wood flour and the highly active cellulose fiber, and subjecting the resultant mixture to a grinding treatment for at least 30 minutes. The above dewatering promoter can be produced at a low cost, and allows the reduction of the cost required for waste water treatments such as the disposal of a great amount of sludge.