PURPOSE: To accelerate formation of compost of brewer's grain and remove offensive odor in production process and obtain a compost useful as an organic fertilizer by mixing the culure fluid of a microorganic group including a specific bacterium with brewer's grain.

CONSTITUTION: A wort solution to which hop is added is inoculated with one or more bacteria belonging to Trichoderma comprising Trichoderma viride and/or Trichoderma lignorum, photosynthetic bacterial which are one or more soil microorganisms selected from among bacterial of the genus Rhodosperillum, bacterial of the genus Rhodopseudomonas and bacterial of the genus Phodobactor, bacteria belonging to lactic acid bacterial which are one or more kinds of soil microorganisms selected from among bacteria of the genus Lactobacillus, bacteria of the genus Pediococcus and bacteria of the genus Leuconostoc and bacteria belonging to nitric acid-reducing bacteria which are one or more kinds of soil mictoorganisms selected from among bacteria of the genus Pseudomonas, bacteria of the genus Paracoccus and bacteria of the genus Aquaspirillum and bacteria of the genus Spirotelium and the cultured product in an amount of 2-4l based on 1m3 brewer's grain is mixed with brewer's grain to ferment the brewer's grain.