This disclosure is to make it possible to easily stabilize a chlorosilane polymer while preventing a solid chlorosilane polymer from being generated. Disclosed is a method for stabilizing a chlorosilane polymer generated secondarily in a step of a chemical vapor deposition method using chlorosilane-based gas, the method including: a step of bringing alcohol into contact with the chlorosilane polymer, degrading the chlorosilane polymer to alkoxide, hydrogen chloride and hydrogen, and diluting the degraded alkoxide with the alcohol; and a step of performing hydrolysis for the alkoxide.