To form a thin Si3N4 film on a wafer surface stably in the CVD process by suppressing the influence of org. deposits on the surface.

A semiconductor device manufacturing apparatus 1 comprises a chamber 3 for applying ozone to a wafer surface under a reduced pressure and chamber 11 for forming an Si3N4 film on the wafer surface. Both chambers are connected under reduced a pressure through a load clock chamber 21. Only applying an ozone gas to the wafer surface easily removes org. compds. as CO2 or H2O gas owing to the oxidizing power of ozone. Thus an Si3N4 thin film can be formed stably by suppressing the time before starting film formation, without being ginfluenced by the org. compds.