The present invention provides an agent for the prophylactic or treatment of diabetes, diabetic complications, insulin resistance syndrome, metabolic syndrome, hyperglycemia, dyslipidemia, atherosclerosis, cardiac failure, cardiomyopathy, myocardial ischemia, brain ischemia, cerebral apoplexy, pulmonary hypertension, hyperlactacidemia, mitochondrial disease, mitochondrial encephalomyopathy or cancer, namely, a PDHK inhibitor and the like. A compound represented by the following formula [I] or a pharmaceutically acceptable salt thereof, or a solvate thereof: wherein each symbol is as defined in the specification.