Allergic bronchopulmonary aspergillosis (ABPA) is a lung disorder caused by a hypersensitivity reaction to antigens of the Aspergillus species. Recently, allergic bronchopulmonary mycosis (ABPM) caused by fungi other than Aspergillus species but with the same symptoms has been described. ABPM commonly affects patients with allergic diseases including bronchial asthma. ABPM is characterized by radiographic appearance, with the most common findings being proximal bronchiectasis and signs of mucoid impaction. However, the differentiation of ABPM is often necessary to enable accurate diagnosis of lung cancer. A 73-year-old man visited the outpatient clinic with symptoms of exertional dyspnea. He was diagnosed with ABPM due to suspicious bronchiectasis and mucoid impaction observed in computed tomography (CT) of his chest. After 3 months, he visited our hospital with continued exertional dyspnea and suspicion of a possible tumor in his lung. Marked eosinophilia and high-attenuation mucus impaction were not taken into consideration as diagnosis was conducted as per clinical diagnostic criteria for ABPA/ABPM. We hereby report a case of lung cancer in a patient initially evaluated for suspected ABPM of the right lung. The diagnosis of lung cancer was established using bronchoscopy. If any definitive diagnosis is not achieved by following the clinical diagnostic criteria for ABPM, physicians should achieve a histological diagnosis by performing a prompt bronchoscopy.