A 57-year-old man was referred to our hospital because of a paranephric tumor detected by preoperative computed tomography (CT) for the ascending colon carcinoma. Since the paranephric tumor was small, surgical treatment was performed only for the colon carcinoma. The follow-up CT showed increase in size and number of paranephric tumor masses. It was suspected to be a soft tissue malignant tumor. Left radical nephrectomy was performed. Pathological findings revealed inflammatory pseudotumor of the paranephric fat. To our knowledge, this is the 4th case with inflammatory pseudotumor of the paranephric fat.