A vapor-phase growth method comprising the steps of introducing a silicon-containing gas and ozone into a reaction vessel containing a sample, and introducing excited oxygen obtained by exciting an oxygen gas or an oxygen-containing gas, into the reaction vessel at the same the as, before, or after the silicon-containing gas and the ozone are introduced into the reaction vessel. The silicon-containing gas and the ozone react, forming an intermediate product which can readily condense. The intermediate product reacts with the excited oxygen, thereby forming a thin insulating film which excels in step coverage and has good insulating property.