Metformin has been widely used for the treatment of type 2 diabetes. However, the effect of metformin on pancreatic β-cells remains controversial. In this study, we investigated the impacts of treatment with metformin on pancreatic β-cells in a mouse model fed a high-fat diet (HFD), which triggers adaptive β-cell replication. An 8-week treatment with metformin improved insulin resistance and suppressed the compensatory β-cell hyperplasia induced by HFD-feeding. In contrast, the increment in β-cell mass arising from 60 weeks of HFD-feeding was similar in mice treated with and those treated without metformin. Interestingly, metformin suppressed β-cell proliferation induced by 1 week of HFD-feeding without any changes in insulin resistance. Metformin directly suppressed glucose-induced β-cell proliferation in islets and INS-1 cells in accordance with a reduction in mTOR signaling. Taken together, metformin suppressed HFD-induced β-cell proliferation independent of the improvement of insulin resistance, partly via direct actions.