We report a case of syndrome of inappropriate secretion of antidiuretic hormone (SIADH) with accompanying severe strongyloidiasis in a 52-year-old male. On admission, he showed drowsiness and emaciation with severe hyponatremia. We gave sodium (saline or salts) in an i.v. drip infusion and orally without improvement. A urinalysis and plasma osmotic pressure test indicated SIADH, therefore, treatment was changed to restrict his sodium intake. The hyponatremia gradually improved initially, but the appetite loss, nausea, and hyponatremia continued. Endoscopy revealed white patches on the stomach wall and histopathological examination revealed infestation of the mucosal epithelium with numerous Strongyloides stercoralis larvae. Ivermectin treatment was then initiated and the abdominal symptoms and hyponatremia gradually resolved. We carefully investigated the underlying cause of the SIADH, such as disease of the central nervous system, lung cancer, and other malignancies, but no abnormality or clear cause could be found. We concluded that the patient developed SIADH secondary to severe S. stercoralis infection.