PROBLEM TO BE SOLVED: To provide a method for producing a mixture of fermented fishmeal and surface-modified steelmaking slag, by which steelmaking slag is used in a process for producing fermented fishmeal using fishery product residues as a raw material, whereby the fermented fishmeal capable of being used as a fertilizer for making seaweed bed with the surface-modified steelmaking slag can simply be produced and supplied at a low cost.SOLUTION: The method for producing a mixture of fermented fishmeal and surface-modified steelmaking slag which is capable of being used as a fertilizer material for making the seaweed, includes mixing fishery residues with steelmaking slag and leaving the mixture, whereby the fishery residues can be fermented and the surface of the steelmaking slag can simultaneously be modified. The steelmaking slag contains slag particles having a particle diameter of not more than 2 mm in an amount of not less than 10 mass%.
