A curable composition comprising polyorganosilsesquioxanes, the polyorganosilsesquioxanes comprising: (a-1) a polyorganosilsesquioxane having a weight-average molecular weight of 3000 or more as determined by GPC and calibrated with a polystyrene standard; and (a-2) a polyorganosilsesquioxane having a weight-average molecular weight of 2500 or less as determined by GPC and calibrated with a polystyrene standard, the polyorganosilsesquioxanes containing the polyorganosilsesquioxane (a-1) and the polyorganosilsesquioxane (a-2) in combination in a total content of 50 weight percent or more of the total amount of the polyorganosilsesquioxanes, the polyorganosilsesquioxanes having a ratio (weight ratio) in content of the polyorganosilsesquioxane (a-1) to the polyorganosilsesquioxane (a-2) of from 10:90 to 70:30, the polyorganosilsesquioxanes (a-1) and (a-2) each independently comprising a constitutional unit represented by Formula: [R 1 SiO 3/2] wherein R 1 represents an epoxy-containing group, the polyorganosilsesquioxanes (a-1) and (a-2) each independently having a mole ratio of a constitutional unit represented by Formula to a constitutional unit represented by Formula of 5 or more, Formulae and expressed as follows: [R a SiO 3/2] wherein R a is selected from an epoxy-containing group, substituted or unsubstituted aryl, substituted or unsubstituted aralkyl, substituted or unsubstituted cycloalkyl, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, and hydrogen, [R b SiO 2/2 (OR c)] wherein R b is selected from an epoxy-containing group, substituted or unsubstituted aryl, substituted or unsubstituted aralkyl, substituted or unsubstituted cycloalkyl, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, and hydrogen; and R c is selected from hydrogen and C 1 -C 4 alkyl, the polyorganosilsesquioxanes (a-1) and (a-2) each independently having a total proportion of the constitutional unit represented by Formula and a constitutional unit represented by Formula of 55 to 100 mole percent of the total amount (100 mole percent) of siloxane constitutional units constituting the polyorganosilsesquioxane (a-1) or (a-2), Formula expressed as follows: [R 1 SiO 2/2 (OR c)] wherein R 1 is as defined in Formula; and R c is as defined in Formula. polyorganosilsesquioxanes, the polyorganosilsesquioxanes comprising: (a-1) a polyorganosilsesquioxane having a weight-average molecular weight of 3000 or more as determined by GPC and calibrated with a polystyrene standard; and (a-2) a polyorganosilsesquioxane having a weight-average molecular weight of 2500 or less as determined by GPC and calibrated with a polystyrene standard, the polyorganosilsesquioxanes containing the polyorganosilsesquioxane (a-1) and the polyorganosilsesquioxane (a-2) in combination in a total content of 50 weight percent or more of the total amount of the polyorganosilsesquioxanes, the polyorganosilsesquioxanes having a ratio (weight ratio) in content of the polyorganosilsesquioxane (a-1) to the polyorganosilsesquioxane (a-2) of from 10:90 to 70:30, the polyorganosilsesquioxanes (a-1) and (a-2) each independently comprising a constitutional unit represented by Formula: [R 1 SiO 3/2] wherein R 1 represents an epoxy-containing group, the polyorganosilsesquioxanes (a-1) and (a-2) each independently having a mole ratio of a constitutional unit represented by Formula to a constitutional unit represented by Formula of 5 or more, Formulae and expressed as follows: [R a SiO 3/2] wherein R a is selected from an epoxy-containing group, substituted or unsubstituted aryl, substituted or unsubstituted aralkyl, substituted or unsubstituted cycloalkyl, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, and hydrogen, [R b SiO 2/2 (OR c)] wherein R b is selected from an epoxy-containing group, substituted or unsubstituted aryl, substituted or unsubstituted aralkyl, substituted or unsubstituted cycloalkyl, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, and hydrogen; and R c is selected from hydrogen and C 1 -C 4 alkyl, the polyorganosilsesquioxanes (a-1) and (a-2) each independently having a total proportion of the constitutional unit represented by Formula and a constitutional unit represented by Formula of 55 to 100 mole percent of the total amount (100 mole percent) of siloxane constitutional units constituting the polyorganosilsesquioxane (a-1) or (a-2), Formula expressed as follows: [R 1 SiO 2/2 (OR c)] wherein R 1 is as defined in Formula; and R c is as defined in Formula. (a-1) a polyorganosilsesquioxane having a weight-average molecular weight of 3000 or more as determined by GPC and calibrated with a polystyrene standard; and (a-2) a polyorganosilsesquioxane having a weight-average molecular weight of 2500 or less as determined by GPC and calibrated with a polystyrene standard,