A 78-year-old man was referred to our department because of an abnormal shadow on the chest radiograph and liver dysfunction after lung resection for lung cancer. Following the operation, loxoprofen sodium was administered to control his chest pain. A chest radiograph on admission showed airspace consolidation in the right lower lung field. In addition, leukocytosis and elevation of CRP were observed. Although piperacillin sodium was given to him, airspace consolidation on a chest radiograph was increased. A bronchoalveolar lavage fluid study showed that total cell counts and proportion of lymphocytes were increased, and the CD4/CD8 ratio was 1.77. A transbronchial lung biopsy specimen revealed organizing pneumonia with Masson bodies. Furthermore, a lymphocyte stimulation test for loxoprofen sodium was positive. From the clinical course, laboratory data and pathologic findings, we considered this case to be loxoprofen sodium-induced BOOP. Withdrawal of loxoprofen sodium and treatment with corticosteroid resulted in marked improvement of the clinical findings. Although a rare occurrence, it is important to recognize that BOOP can be caused by loxoprofen sodium.