Abstract: The present invention aims to provide a novel compound having a high mPTP-opening inhibitory activity and/or a useful therapeutic effect on various diseases. An aspect of the present invention relates to a mitochondrial permeability transition pore (mPTP)-opening inhibitor, a medicament or a pharmaceutical composition comprising a compound represented by formula (I), wherein R1, R2, R3, R4, R5, R6, R7, R8, R9, R10 and R11 are the same as defined in the specification and the claims, a stereoisomer thereof, a prodrug thereof, or a pharmaceutically acceptable salt thereof, or a pharmaceutically acceptable solvate thereof, as an active ingredient. Another aspect of the present invention relates to a compound represented by formula (Ia), wherein R1a, R2, R3, R4, R5, R6, R7, R8, R9, R10 and R11 are the same as defined in the specification and the claims, a stereoisomer thereof, a prodrug thereof, or a salt thereof, or a solvate thereof.