Nephrotic syndrome (NS) is one of the common pediatric diseases that require glucocorticoid treatment. Patients with NS might receive steroids for a long time if remission is not achieved. Evidence shows that long-term steroid use may induce osteoporosis in adults and children, and steroid use is well known to be related to avascular necrosis of the femoral head (ANFH) in adults. However, no pediatric case of AFNH caused by long-term steroid use due to NS has been reported. In this report, we describe the case of a three-year-old boy with a chief complaint of gait difficulty, who had been treated with glucocorticoid orally for a year because of NS. His body temperature was within the normal limit. His legs did not show trauma, redness, or swelling; however, he did not want his left thigh touched. A pelvic X-ray scan showed asymmetrical femoral heads due to the thinning of the left femoral head. Pelvic magnetic resonance imaging showed a low intensity of the left femoral head on the T2-weighted image and high and low mixed intensities on the fat-suppressed T2-weighted image. Deformation of the left femoral head was suspected. The epiphysial nucleus of the right femoral head was also small for his age. He was diagnosed with Legg-Calvé-Perthes disease and referred to an orthopedic clinic to begin rehabilitation with equipment to support his joints. Thus, we cannot completely conclude that glucocorticoid use and NS are not related to AFNH in children. Physicians must consider early diagnosis.