A 67-year-old woman was admitted to our hospital with polyarticular pain, fever, and dry cough that were refractory to corticosteroid therapy. Computed tomography of the chest showed a huge mass with cavity formation in the upper lobe of the left lung and multiple bilateral lung nodules. Pathological examination of specimens obtained by transbronchial lung biopsy from the left-sided S³ revealed necrotizing granulomatous vasculitis. Laboratory findings showed an elevated serum PR3-anti-neutrophil cytoplasmic antibody (ANCA) level. On the 7th day after admission, she developed rapidly progressive glomerulonephritis with a rapid decline in renal function and an increase in protein and occult blood in the urine. Based on these findings, symptoms, and the clinical course, the patient was diagnosed as having granulomatosis with polyangiitis (GPA). On receiving steroid pulse therapy, plasma exchange, and oral cyclophosphamide, her symptoms improved dramatically, and the lung mass and multiple nodules shrank remarkably. Often, GPA cases presenting as a lung mass are misdiagnosed as either lung cancer or a lung infection. Therefore, this case demonstrates the importance of considering the possibility of GPA at the time of examination of the lung mass in patients with complaints of polyarticular pain and/or fever of unknown etiology.