The invention relates to a convenient screening tool for identifying an agent for treating diabetes. The screening tool is a G protein-coupled receptor, a variant of the G protein-coupled receptor, or a homolog of the G protein-coupled receptor, which promotes insulin secretion under a high glucose concentration by activation. The invention also relates to cells transformed with an expression vector comprising a polynucleotide encoding and expressing the screening tool. The invention provides a convenient screening method for identifying an agent for treating diabetes, pharmaceutical compositions comprising the agent, and a process for manufacturing the pharmaceutical composition.