1. A laddery lower alkylpolysilsesquioxane having a heat resistant thin film-formability represented by the formula: ##STR3## wherein R and R' represent same or different lower alkyl groups, and n is an average degree of polymerization, characterized in that 15 to 30% by weight of the laddery lower alkylpolysilsesquioxane is occupied by the low molecular weight portion having a standard polystyrene-reduced molecular weight of 20,000 or less as measured by gel permeation chromatography.