A copolymerized polysilazane comprising at least repeating units represented by general formula (I): —Si(R1)(R2)—NR3— and repeating units represented by general formula (II): —Si(R1)(R2)—NH— (in the formulas, R1 and R2 each independently represent a hydrogen atom, hydrocarbon group, hydrocarbon group-containing silyl group, hydrocarbon group-containing amino group, or hydrocarbon oxy group, and R3 represents an alkyl group, alkenyl group, alkoxy group, cycloalkyl group, aryl group or alkyl silyl group), and the NR3/SiH1,2 ratio (SiH1,2 represents the total amount of SiH1 and SiH2) is 0.005-0.3. Said copolymerized polysilazane can be manufactured by reacting Si(R1)(R2)X2 (in the formula, X represents a halogen atom) with a primary amine compound: R3NH2 and then reacting with ammonia, and is able to form a siliceous film that has withstand voltage characteristics and solvent resistance by curing at a low temperature.