A Japanese boy aged 9 years referred to our hospital because of steroid-resistant proteinuria. He had a 6-year history of unremitting proteinuria and was diagnosed as having minimal-change disease (MCD) by the repeated renal biopsies performed at the age of 3.5 years and 8.5 years, respectively. His proteinuria fluctuated ranging from 115 mg/100 ml to 645 mg/100 ml, and serum total protein ranged from 59 g/liter to 63 g/liter. The third renal biopsy at the presentation also revealed MCD. Thereafter he was treated with an anti-thrombocyte agent combined with an angiotensin converting enzyme inhibitor. Despite unremitting proteinuria of long duration, he did not have any complaints. At the age of 11.5 years, severe tubulointerstitial lesion was observed in the fourth renal biopsy. The fifth renal biopsy 6 months after the fourth finally revealed the lesion of focal segmental glomerulosclerosis (FSGS). Although the interpretation of his repeated renal biopsies were considered to be limited, these clinical observation suggested that his unremitting proteinuria of long duration might have been attributed to subsequent progression FSGS.