Objective: We clarified the results of the incidence of an adverse event (AE) in the ICU at a university hospital. Methods: Using a random sampling method, 88 patients were selected, and we retrospectively reviewed their medical records using the Global Trigger Tool. Results: During the study period, 119 AEs occurred. A total of 57.3% of the patients experienced AEs, and the incidence of occurrence was 300.5 AEs for every 1,000 patient days and approximately 135.2 AEs per 100 ICU admissions. The amount of preventable AEs was determined to be 39.2%. The most frequent AE was in the cardiovascular department. The most frequent cause of AE was medications. For the level of harm, 3a (temporary harm to the patient and required intervention) was 63.9%; 3b (temporary harm to the patient and required initial or prolonged hospitalization) was 33.6%; 4a (permanent patient harm) was 0.0%; 4b (intervention required to sustain life) was 0.8%; 5 (patient death) was 1.7%. Conclusion: The incidence of AEs in the ICU was 57.3%. About 60% of the AEs were mild, and about 40% of the AEs were preventable.