Background: The prognosis of steroid resistant nephrotic syndrome (SRNS) in children is poorer than steroid-sensitive cases. The diagnosis is made following the response to the initial 4-week corticosteroid therapy, which might be accompanied by side effects. However, predictive indicators at initial diagnosis remain unknown. We aimed to investigate whether selectivity index (SI) and other indicators at initial diagnosis, for example, serum immunoglobulin M (IgM) and total serum protein albumin ratio (TA ratio, total serum protein level over albumin level), can predict SRNS. Methods: Eighty children were enrolled from seven hospitals in Japan between January 2008 and December 2019 (mean age 4.7; male 65.0 %). Of the enrolled 80 children, 13 (16.3 %, M:F = 5:8) had been diagnosed as steroid-resistant after the initial treatment with steroids. The association between serum IgM (tertile categories; low: 24-133; middle: 134-169; and high: 169.1-510 mg/dl), SI (<0.2 or ≥0.2), and TA ratio (tertile categories; low: 1.8-2.6; middle: 2.62-3.75; and high: 3.8-15.3) at initial diagnosis and steroid resistance was evaluated with logistic regression adjusting for age and sex. Results: Low levels of serum IgM were significantly associated with steroid resistance (adjusted odds ratio (aOR) = 6.94; 95% confidence interval (CI): 1.12-43.11). TA ratio and SI were not significantly associated with steroid resistance. Conclusions: Low levels of serum IgM at initial diagnosis might predict steroid resistance among Japanese children with idiopathic nephrotic syndrome.