The etiology of erythema multiforme has not been fully understood to date, but it is generally considered to be a skin disease caused by a metabolic reaction. In recent years, the relationship between the disease and herpes simplex virus has attracted considerable attention. It has been pointed out that more than 30% of patients with erythema multiforme are caused by the herpes virus, and this has been confirmed by tests in which intradermal injections with herpes simplex virus antigen can cause the development of the disease. In addition, some cases are often drug rashes caused by sulfonamide preparations, antipyretic and analgesic drugs, sedative and sleeping agents, antibiotics, etc.; in some patients, the onset is related to chronic focal infections, such as tonsillitis, otitis media, paranasal sinusitis, chronic adnexitis, etc.; some are one of the clinical symptoms of internal diseases, such as rheumatic fever, typhoid fever, wave fever, infectious mononucleosis, malignant granuloma and various Some of them are related to clinical symptoms of internal diseases, such as rheumatic fever, typhoid fever, wave fever, infectious mononucleosis, malignant granuloma and various malignant tumors, etc.; some of them are caused after consuming daily foods such as fish and eggs, etc.; there are also a considerable number of cases whose causes are still difficult to identify.