Provided is a heterocyclic compound having a superior RBP4-lowering action and useful as a medicament for the prophylaxis or treatment of a disease or symptom mediated by an increase in RBP4 or retinol supplied by RBP4.A compound represented by the formula (I): wherein each symbol is as defined in the Description, or a salt thereof has a superior RBP4-lowering action, and is useful as a medicament for the prophylaxis or treatment of a disease or symptom mediated by an increase in RBP4 or retinol supplied by RBP4.