A 37-year-old woman at 17 weeks of gestation who was first noted to have proteinuria and microscopic hematuria at 13 weeks of gestation was admitted to our hospital with proteinuria that progressed to nephrotic syndrome (NS). Despite the treatment with prednisolone, including methylprednisolone pulse therapy, the NS worsened. The patient underwent an elective abortion at 21 weeks of gestation, and the NS then went into partial remission. A renal biopsy revealed membranous nephropathy (MN). There was no evidence of secondary MN. This is the first reported case of subclinical idiopathic MN that first developed in pregnancy.