The present invention relates to a medicament comprising a compound represented by the formula (I) wherein R 1 is a group represented by the formula wherein R 2 , R 3 , R 4 , R 5 , R 6 , R 7 and R 8 are each independently a hydrogen atom or a C 1-6 alkyl, or a salt thereof, which is an agent for the prophylaxis or treatment of diseases in which insulin resistance is involved.