An insulation film is formed on a semiconductor substrate by vaporizing a silicon-containing hydrocarbon compound to provide a source gas, introducing a reaction gas composed of the source gas and an additive gas such as an inert gas and oxidizing gas to a reaction space of a plasma CVD apparatus. In particular, the present invention relates to a method for reducingsuppressing sheeting or agglomerates during the product grade transition andor catalyst transitions occuring polymerisation of olefins. Each of the first switches is connected between the first potential terminal and one of the first connecting points and the first node.