A 65-year-old woman, in whom bilateral reticular shadows were detected on a chest radiograph at medical examination, was referred to our hospital for detailed examination. She had been complaining of weight loss, eruption and finger stiffness. Physical examination revealed bilateral lower leg edema and erythema on the left lower leg. Computed tomography of the chest showed bilateral perivascular ground-glass opacities and consolidations. Surgical lung biopsy was not performed. However, a biopsy specimen obtained from erythema on the left lower leg showed the findings of leukocytoclastic vasculitis. The myeloperoxidase anti-neutrophil cytoplasmic antibody (MPO-ANCA) test was negative at the first visit, but, on subsequent examination, the MPO-ANCA test was positive, and microscopic polyangiitis (MPA) was diagnosed. She was successfully treated with systemic corticosteroids and cyclophosphamide. This is a rare case of unclassifiable interstitial pneumonia associated with MPA.