The case is a 51-year-old man whose bronchial asthma and allergic rhinitis had been treated for more than ten years and whose ulcerative colitis (UC) had been treated with mesalazine for one year. His UC deteriorated and his symptoms were improved by the administration of prednisolone (PSL) and azathioprine (AZP). After the cessation of PSL and AZP, he presented with wheezing, fever, eosinophilia, sinusitis, and multiple infiltrates on his chest computed tomography. Histopathological findings from the transbronchial lung biopsy specimens showed a large number of eosinophils in the alveolar spaces. The collections of macrophages included multinucleated giant cells indicating the presence of granulomas, and occlusion of the vascular lumen and rupture of the elastic fiber in the peripheral pulmonary artery demonstrated vasculitis. He was diagnosed with eosinophilic granulomatosis with polyangiitis (EGPA). PSL 40 mg was administered and resulted in prompt improvement, and his symptoms and the infiltrates on computed tomography disappeared in one month. After that PSL was tapered to 10 mg and, up until the time of writing, there has been no deterioration in EGPA or UC symptoms. Although reports of EGPA associated with UC are rare, it is important to be alert for complications while these diseases are treated.