Film-forming apparatus including a film-forming vacuum chamber having a stage for a substrate, a chamber for mixing gas comprising a raw gas and a reactive gas connected to the film-forming chamber, a chamber for vaporizing the raw material, and a gas head for introducing the mixed gas into the film-forming chamber, disposed on the upper face of the film-forming chamber and opposed to the stage. Particle traps with controllable temperatures are positioned between the vaporization chamber and the mixing chamber and on the downstream side of the mixing chamber. When forming a thin film with the apparatus, a reactive gas and/or a carrier gas are passed through the film-forming chamber while opening a valve in a by-pass line, connecting the primary side to the secondary side of the particle trap arranged at the downstream side of the mixing chamber. The valve is then closed and the film-forming operation is initiated.