[Problem] To provide a polycarbosilazane making it possible to form a silicon-containing film which is bearable to acid etching, and a composition comprising the polycarbosilazane. [Means for Solution] The present invention provides a polycarbosilazane comprising a repeating unit of —[R1R2Si—(CH2)n]— and —(R3R4Si—NR5)—, wherein R1, R2, R3 and R4 are each independently a single bond, hydrogen or C1-4 alkyl; R5 is independently a single bond or hydrogen; and n is 1-2, and a composition comprising the polycarbosilazane. The present invention also provides a method for forming a silicon-containing film, comprising coating the composition above a substrate and heating.