By enhancing the function of ERK proteins in the liver, proliferation of pancreatic β cells is promoted, blood insulin concentration increased, blood glucose level decreased, and diabetes is prevented and/or treated. The methods for enhancing the function of ERK proteins in the liver are not particularly limited, and include various aspects such as enhancement of activity of MEK proteins, activation of endogenous MEK proteins, administration of an expression vector which expresses a gene encoding an active-form of MEK protein, and the like.
