A 71-year-old man with primary adenocarcinoma of the lung in the left upper lobe (clinical stage IV) was undergoing chemotherapy when bilateral adrenal metastases appeared 2 years ago. During treatment with atezolizumab, he was hospitalized with lower abdominal pain, diarrhea, and oral mucosal pigmentation. We diagnosed him with Addison's disease due to low cortisol levels and high ACTH levels. We considered the cause of the Addison's disease to be either bilateral adrenal metastasis of lung cancer or atezolizumab. Dynamic adrenal size changes after treatment for primary adrenalcortical insufficiency can be a characteristic of Addison's disease caused by immune checkpoint inhibitors. In this case, no adrenal contractions were observed after hydrocortisone replacement therapy, and the cause was determined to be adrenal metastasis. Immune checkpoint inhibitors can be continued with hormone replacement in both adrenal metastasis and immune-related adverse events. However, the mechanism and risk factors for endocrine disorders secondary to immune checkpoint inhibitors are unknown. In many cases, it may be complicated by other endocrine diseases, so it is important to distinguish between the two causes of the Addison's disease.