We report a 14-year-old man with Crohn's disease (CD) who developed right upper arm pain while being treated with the anti-tumor necrosis factor (TNF)-alpha monoclonal antibody, infliximab. There were no symptoms suggestive of active CD, but the inflammatory response was high, and a contrast-enhanced CT showed the occlusion of the right brachial artery. We diagnosed the patient as having Takayasu's arteritis (TA) and started treatment with corticosteroids, then tapered off the steroids as the symptoms of TA resolved. Later, TA flared up, and his treatment was changed from infliximab to an anti-IL-6 receptor antibody, tocilizumab. The change to TCZ stabilized TA, but exacerbated CD. It is difficult to control both diseases at the same time, and the choice of biologics for treatment must be carefully considered.