Recently, there are several reports of simultaneous allergic bronchopulmonary aspergillosis (ABPA) and Mycobacterium-avium complex (MAC) lung disease. However, the strategies for early diagnosis and appropriate treatment for patients with both ABPA and MAC lung disease have not been established. Here, we report a case with ABPA complicated by MAC lung disease, which was successfully diagnosed and treated by simultaneous administration of systemic steroids and antimycobacterial drugs. Bronchoscopy can be useful in the diagnosis of such cases. Furthermore, in a patient with concurrent ABPA and MAC lung disease, simultaneous treatments for both diseases could reduce both diseases.