We encountered a case of primary lung cancer complicated with membranous nephropathy as primary nephrotic syndrome. Because treatment approaches vary greatly for primary and secondary nephrotic syndrome, a renal biopsy was performed for diagnosis. Much time was required to make a definitive diagnosis of primary nephrotic syndrome, as opposed to paraneoplastic nephrotic syndrome. Consequently, the subsequent chemotherapy was ineffective and caused significant toxicity due to reduced performance status (PS) and progression of hypoalbuminemia. Therefore, it is imperative that a diagnosis be made and treatment be initiated without delay before PS declines and hypoalbuminemia progresses.