A curable composition for lens formation, the curable composition comprising: a cycloaliphatic epoxide represented by Formula (a); a cationic-polymerization initiator; a polysiloxane represented by Formula (c), a siloxane compound containing two or more epoxy groups per molecule; and a cationically curable compound containing one or more hydrogenated glycidyl ether epoxide groups per molecule, excluding compounds belonging to the cycloaliphatic epoxide and the siloxane compound; wherein the curable composition contains the polysiloxane in an amount of 0.01% to 5% by weight based on the total amount (100% by weight) of the curable composition; the content of an optional additive is 5% by weight or less based on the total amount (100% by weight) of the curable composition; and Formulae (a) and (c) are expressed as follows: wherein: R 1 to R 18 are each, identically or differently, selected from hydrogen, halogen, a hydrocarbon group which may contain oxygen or halogen, and optionally substituted alkoxy; and X is selected from a single bond and a linkage group excluding a siloxane-bond-containing group, wherein: R 19 to R 22 each represent, identically or differently in each occurrence, a group selected from hydrogen, alkyl, haloalkyl, aryl, aralkyl, alkoxy, acyloxy, and -RNHCOR', where R is selected from alkylene and alkenylene, and R' is selected from alkyl and alkenyl; and m and n each represent, identically or differently, an integer of 1 or more. a cycloaliphatic epoxide represented by Formula (a); a cationic-polymerization initiator; a polysiloxane represented by Formula (c), a siloxane compound containing two or more epoxy groups per molecule; and a cationically curable compound containing one or more hydrogenated glycidyl ether epoxide groups per molecule, excluding compounds belonging to the cycloaliphatic epoxide and the siloxane compound; wherein the curable composition contains the polysiloxane in an amount of 0.01% to 5% by weight based on the total amount (100% by weight) of the curable composition; the content of an optional additive is 5% by weight or less based on the total amount (100% by weight) of the curable composition; and Formulae (a) and (c) are expressed as follows: wherein: R 1 to R 18 are each, identically or differently, selected from hydrogen, halogen, a hydrocarbon group which may contain oxygen or halogen, and optionally substituted alkoxy; and X is selected from a single bond and a linkage group excluding a siloxane-bond-containing group, wherein: R 19 to R 22 each represent, identically or differently in each occurrence, a group selected from hydrogen, alkyl, haloalkyl, aryl, aralkyl, alkoxy, acyloxy, and -RNHCOR', where R is selected from alkylene and alkenylene, and R' is selected from alkyl and alkenyl; and m and n each represent, identically or differently, an integer of 1 or more. R 1 to R 18 are each, identically or differently, selected from hydrogen, halogen, a hydrocarbon group which may contain oxygen or halogen, and optionally substituted alkoxy; and X is selected from a single bond and a linkage group excluding a siloxane-bond-containing group, wherein: R 19 to R 22 each represent, identically or differently in each occurrence, a group selected from hydrogen, alkyl, haloalkyl, aryl, aralkyl, alkoxy, acyloxy, and -RNHCOR', where R is selected from alkylene and alkenylene, and R' is selected from alkyl and alkenyl; and m and n each represent, identically or differently, an integer of 1 or more. R 19 to R 22 each represent, identically or differently in each occurrence, a group selected from hydrogen, alkyl, haloalkyl, aryl, aralkyl, alkoxy, acyloxy, and -RNHCOR', where R is selected from alkylene and alkenylene, and R' is selected from alkyl and alkenyl; and m and n each represent, identically or differently, an integer of 1 or more.