A polymerizable composition comprising: (a) 100 parts by mass of a reactive silsesquioxane compound, which is a polycondensate of an alkoxy silicon compound A of formula and an alkoxy silicon compound B of formula: Ar 1 -Si(OR 1) 3 wherein Ar 1 is a phenyl group having at least one group with a polymerizable double bond, a naphthyl group having at least one group with a polymerizable double bond, or a biphenyl group having at least one group with a polymerizable double bond; and R 1 is methyl group or ethyl group; Ar 2 -Si(OR 2) 3 wherein Ar 2 is a phenyl group optionally substituted with a C 1-6 alkyl group, a condensed polycyclic aromatic hydrocarbon group optionally substituted with a C 1-6 alkyl group, or a hydrocarbon ring assembly group (optionally substituted with a C 1-6 alkyl group) in which a plurality of aromatic rings are directly joined by a single bond; and R 2 is methyl group or ethyl group; (b) 10 to 500 parts by mass of a fluorene compound of formula: wherein R 3 and R 4 are each independently a hydrogen atom or methyl group; L 1 and L 2 are each independently a phenylene group optionally having a substituent or a naphthalenediyl group optionally having a substituent; L 3 and L 4 are each independently a C 1-6 alkylene group; and m and n are each zero or a positive integer such that m + n is 0 to 40; and (c) 1 to 100 parts by mass of an aromatic vinyl compound of formula: wherein R 5 is a hydrogen atom or methyl group; L 5 is a single bond, a hydrogen atom, an oxygen atom, a C 1-20 aliphatic hydrocarbon residue having a valence of q optionally substituted with phenyl group, or a C 1-20 aliphatic hydrocarbon residue having a valence of q optionally containing an ether bond; Ar 3 is an aromatic hydrocarbon residue having a valence of p + 1; p's are each independently 1 or 2; and q is an integer from 1 to 3 with the proviso that q is 1 when L 5 is a hydrogen atom, and q is 2 when L 5 is a single bond or an oxygen atom. (a) 100 parts by mass of a reactive silsesquioxane compound, which is a polycondensate of an alkoxy silicon compound A of formula and an alkoxy silicon compound B of formula: Ar 1 -Si(OR 1) 3 wherein Ar 1 is a phenyl group having at least one group with a polymerizable double bond, a naphthyl group having at least one group with a polymerizable double bond, or a biphenyl group having at least one group with a polymerizable double bond; and R 1 is methyl group or ethyl group; Ar 2 -Si(OR 2) 3 wherein Ar 2 is a phenyl group optionally substituted with a C 1-6 alkyl group, a condensed polycyclic aromatic hydrocarbon group optionally substituted with a C 1-6 alkyl group, or a hydrocarbon ring assembly group (optionally substituted with a C 1-6 alkyl group) in which a plurality of aromatic rings are directly joined by a single bond; and R 2 is methyl group or ethyl group; Ar 1 is a phenyl group having at least one group with a polymerizable double bond, a naphthyl group having at least one group with a polymerizable double bond, or a biphenyl group having at least one group with a polymerizable double bond; and R 1 is methyl group or ethyl group; (b) 10 to 500 parts by mass of a fluorene compound of formula: wherein R 3 and R 4 are each independently a hydrogen atom or methyl group; L 1 and L 2 are each independently a phenylene group optionally having a substituent or a naphthalenediyl group optionally having a substituent; L 3 and L 4 are each independently a C 1-6 alkylene group; and m and n are each zero or a positive integer such that m + n is 0 to 40; and (c) 1 to 100 parts by mass of an aromatic vinyl compound of formula: wherein R 5 is a hydrogen atom or methyl group; L 5 is a single bond, a hydrogen atom, an oxygen atom, a C 1-20 aliphatic hydrocarbon residue having a valence of q optionally substituted with phenyl group, or a C 1-20 aliphatic hydrocarbon residue having a valence of q optionally containing an ether bond; Ar 3 is an aromatic hydrocarbon residue having a valence of p + 1; p's are each independently 1 or 2; and q is an integer from 1 to 3 with the proviso that q is 1 when L 5 is a hydrogen atom, and q is 2 when L 5 is a single bond or an oxygen atom.