A composition comprising: a polymerized substance of a compound (I) that contains m numbers of RSi(O0.5)3 units, wherein m represents an integer of from 8 to 16; and R's each independently represents a non-hydrolysable group, provided that at least two among R's represent groups containing a vinyl group or an ethynyl group, and wherein each one of the RSi(O0.5)3 units is connected to another one of the RSi(O0.5)3 units by sharing an oxygen atom in each one of the RSi(O0.5)3 units, so as to form a cage structure, and wherein within a solid component contained in the composition, a polymerized substance formed by a reaction of the compound (I) represents 60 mass % or more.