PURPOSE: To prevent the generation of an undesired film deposition and fine grains generated on the inner surface of the light incidence window of a reaction device by a method wherein a substrate is irradiated with excitation light in a highly efficient manner by reflecting it on the side face of the gap of a separator, and the conductance of reaction gas and the gas which does not contribute to the reaction is properly controlled using the gap located between the separators.

CONSTITUTION: Excitation light 1 becomes 1a which reaches a substrate directly passing through a gap, or 1b which reaches the substrate after reaction on the sidewall of the gap of a separator. As a result, the excitation light is not shielded by the separator, and it can be made to irradiate on the substrate efficiently. Also, as the conductance of gas based on the gap is determined by the thickness (x) of the separator, the size of the gap (y) and the angle of the gap (θ), the mixture of the reactive gas and the gas which does not reactively controlled can be prevented, and the generation of turbulence of reaciton gas stream can also be prevented. As a result, the illuminance of excitation light on the reaction system, especially on the substrate is not reduced, the turbulence of the reaction gas stream is small, and an optical CVD device, having excellent cloudiness preventing effect of the light incidence window, can be obtained.