An 80-year-old man presented with a mildly decreased renal function and increased anti-double-stranded-DNA (anti-dsDNA) antibody levels, and met the diagnostic criteria of the American College of Rheumatology for systemic lupus erythematosus (SLE). However, the incremental increase in creatinine levels and the mild proteinuria were inconsistent with lupus nephritis. We performed a renal biopsy, which revealed interstitial nephritis and minor glomerular abnormalities. Further examinations determined that the renal lesion was due to Sjögren's syndrome secondary to SLE. Following treatment with oral prednisolone, the patient's renal function improved as his anti-dsDNA antibody levels decreased. This case report indicates that renal biopsy should be considered even in elderly individuals when it may assist in the diagnosis, treatment, and prognosis of the patient.