Immune-related adverse events (irAEs) occur in rare cases, even after the completion of immune checkpoint inhibitor (ICI) therapy. We encountered a lung cancer patient diagnosed with acute-onset type 1 diabetes mellitus (DM) 7 months after the cessation of ICI. A 68-year-old woman was referred to our hospital for chest abnormalities. She was diagnosed with lung adenocarcinoma cT4N2M1c, stage IVB. Immunostaining showed that the expression of programmed death ligand 1 in tumor cells was negative. A genetic analysis using the Oncomine Dx Target Test Multi-CDx System revealed that the primary tumor was positive for ERBB2. Combined immunotherapy with carboplatin, pemetrexed, and pembrolizumab was performed as first-line therapy, followed by maintenance therapy with pemetrexed plus pembrolizumab, which was successful. After the seventh course, maintenance therapy was stopped because only the primary tumor showed local enlargement. Local chest radiotherapy (66 Gy/33 Fr) was performed, and the patient was followed up. HbA1c was 4.9% 3 months after the completion of pembrolizumab, and dry mouth and polyuria occurred after 5 months. Seven months later, the patient developed diabetic ketoacidosis with a blood glucose of 348 mg/dL and an HbA1c of 11.3%. Antiglutamic acid decarboxylase antibodies were negative and urinary C-peptide was 9.3 μg/day. The patient was diagnosed with acute-onset type 1 diabetes and received insulin therapy. There has been no case report of type 1 diabetes diagnosed 7 months after the last administration of an ICI. These results indicate that irAE needs to be considered even after the cessation of ICI.