The present invention provides such a three-dimensional ladder polymer that has, as a main chain, a silsesquioxane skeleton having a structure of the cage structure of octasilsesquioxane with two broken corners. The polymer of the invention is a polymer obtained by using a silsesquioxane derivative represented by formula (1-0). R0 each represents hydrogen, alkyl, cycloalkyl, aryl or arylalkyl, R1 each represents chlorine, a group defined as similar to R0, or a group having —CN, X independently represents hydrogen, chlorine, a group defined as similar to R1, or a group having one of —CH═CH—, —C≡C—, —OH, —COOH, —COO—, 2-oxapropane-1,3-dioyl, oxiranyl, oxiranylene, oxetanyl, oxetanylene, 3,4-epoxycyclohexyl, —SH, —NCO, —NH— and —NH2; and at least two of X each is a reactive group.