To obtain straw treated with an effective microorganism group, capable of controlling growth of molds harmful to domestic animals without deteriorating qualities of straw and treating the straw in a short time without being affected by weather, useful for a feed, etc., by spraying a straw with an effective microorganisms.

A straw such as rice straw, wheat straw, etc., is sprayed with effective microorganisms capable of removing molds harmful to domestic animals and harmless to human body, e.g. lactic acid bacteria belonging to the genus Lactobacillus, Propionibacterium, etc., photosynthetic bacteria such as Rhodoseudomonas, Rhodospirillum, etc., filamentous fungi belonging to the genus Mucor such as Mucor hiemalis, etc., actinomycetes belonging to the genus Streptomyces, Streptoverticillium, etc., yeast belonging to the genus Saccharomyces, Candida, etc. Preferably the straw is treated by spraying the straw with the effective microorganisms.