Ulcerative colitis (UC) can be complicated by extraintestinal manifestations; for example, arthritis, pyoderma gangrenosum, and uveitis. On the other hand, several cases of interstitial pneumonia accompanying UC have been reported. Case: A 75-year-old man with a history of UC who had been treated with mesalazine since 20XX-3. In February 20XX, he was hospitalized for 2 months because of exertional dyspnea. A chest computed tomography (CT) scan revealed diffuse consolidation in both lungs and a blood test showed elevated KL-6 serum levels (844U/mL). We suspected interstitial pneumonia and performed a bronchoscopy. Bronchoalveolar lavage fluid (BALF) analysis showed a high lymphocyte count (44%), and bacterial infection was ruled out. We diagnosed him with a drug-induced lung injury due to mesalazine, and discontinued mesalazine. The patient experienced a favorable response to steroid therapy. However, while tapering the steroid, lung consolidation occurred again, and we evaluated the use of the steroid. In addition, lung consolidation had not recurred after an incidental resumption of mesalazine. From this clinical course, we confirmed interstitial pneumonia accompanying UC rather than a drug-induced lung injury. We report a case of interstitial pneumonia accompanying UC that was barely distinguishable from drug-induced lung injury caused by mesalazine.