A case of ringworm in horse was observed on a livestock farm in Ibaragi Prefecture. Clinical and mycological examinations revealed that this ringworm was a Microsporum gypseum infection, which has rarely been observed among animals in Japan. The morphological characteristics of this fungus were described. Guinea pigs were susceptible to this isolate. Arthrospores were seen mainly in crusts collected from the infected lesion, forming short chains and aggregations.