The invention provides a film-forming apparatus by a photo-CVD method comprising such essential structural elements as a light inlet in the form of a tube, a film-forming chamber, a light outlet in the form of a tube and a nozzle for feeding a mixture of a reactant and a gas for dilution so that a specific relation exists in the dimensions between the elements.With this structure, the film-forming apparatus by a photo-CVD method prevents the blur of the laser beam-incoming window due to contaminants adhered thereto, thereby enabling the extension of reaction time, increase of the deposition, and formation of a film with uniform thickness, and making the composition of the film closest to the stoichiometric one.