A previously healthy 44-year-old Japanese man with a 5-month history of lumbago presented to the emergency department with acute respiratory failure caused by pneumonia, and was immediately intubated. Computed tomography revealed a lung mass, pleural effusion, and multiple osteolytic lesions; however, the results of thoracentesis and bronchial brushing were not definitive. We performed a bone tumor biopsy guided by diffusion-weighted magnetic resonance imaging (DW-MRI) with mechanical ventilation, which enabled the diagnosis of ALK rearrangement-positive lung adenocarcinoma. In the era of precision medicine requiring proper biological tissue collection, DW-MRI was critical for identifying the biopsy site safely and with high precision.