We performed a randomized crossover trial to investigate the effects of ingesting vegetables before or after a mixed meal and a mixed meal alone on postprandial interstitial fluid glucose levels in patients with type 2 diabetes mellitus, using flash glucose monitoring (FGM). When vegetables were ingested before a mixed meal, interstitial fluid glucose levels were significantly lower at 40-90 minutes after the start of ingestion, and the glucose incremental area under the curve (IAUC) values from 0 to 60, 90, and 120 minutes were significantly lower in comparison to when vegetables were consumed after a mixed meal (p< 0.05). In addition, glucose levels were also significantly lower at 40 and 55-70 minutes after the start of ingestion, and the IAUC from 0-90 minutes was significantly lower, in comparison to when a mixed meal was ingested alone (p< 0.05). However, there was no difference in the postprandial interstitial fluid glucose levels when vegetables were consumed after a mixed meal in comparison to when a mixed meal was consumed alone. In conclusion, our results suggest that when patients with type 2 diabetes mellitus eat a mixed meal, ingesting vegetables before the mixed meal may suppress the elevation of postprandial blood glucose levels.