A 66-year-old man developed multiple erosions and pain in the lips and mouth, fever, and black stools. There was persistent bleeding from the lip erosions. When he was admitted to our hospital, his white blood cell count increased to 53,420/µl with 3% eosinophils, and hemoglobin decreased to 3.1 g/dl. Bone marrow biopsy revealed an elevated eosinophil level (24.0%) with markedly toxic granules. Gastrointestinal endoscopy revealed multiple ulcers and erosions in the pharynx, esophagus, stomach, and colon. Histopathological diagnosis indicated nonspecific inflammation with poor infiltration of eosinophils. Bone marrow FISH test was positive for 4q12 deletion (FIP1L1::PDGFRA), leading to the diagnosis of FIP1L1::PDGFRA-positive chronic eosinophilic leukemia. Following initiation of oral administration of imatinib 100 mg/day, the number of eosinophils decreased rapidly, and normalized 2 days after the start of imatinib. The mucosal lesions showed significant improvement and were diagnosed as leukemia-associated lesions. Based on the clinical course of our patient, multiple oral cavity and gastrointestinal ulcers could be the initial presentation in this leukemia.