A heating medium composition for solar thermal power generation system, the heating medium composition including a silane coupling agent represented by formula (1) shown below and a heating medium containing diphenyl ether: (1) wherein each of OR1, OR2 and OR3 may be the same or different, and represents an alkoxy group of 1 to 5 carbon atoms, and X is a group selected from a 3-glycidoxypropyl group, a 3-methacryloxypropyl group, a 3-aminopropyl group, an N-phenyl-3-aminopropyl group and an N-2-(aminoethyl)-3-aminopropyl group.