Provided is a curable composition having excellent film-formability and can be cured at low temperatures to form a cured product that has heat resistance, cracking resistance, and adhesiveness and adhesion to an adherend at excellent levels. The curable composition according to the present invention is a composition containing polyorganosilsesquioxanes (A). The polyorganosilsesquioxanes (A) include (a-1) an epoxy-containing polyorganosilsesquioxane having a weight-average molecular weight of 3000 or more, and (a-2) an epoxy-containing polyorganosilsesquioxane having a weight-average molecular weight of 2500 or less. The polyorganosilsesquioxanes (A) contain the components (a-1) and (a-2) in a total content of 50 weight percent or more of the total amount of the polyorganosilsesquioxanes (A). The polyorganosilsesquioxanes (A) have a ratio in content (weight ratio) of (a-1) to (a-2) of from 10:90 to 70:30.